Religion and charity may have marched hand in hand in Victorian times, but surely in this modern world, they don't have to. Those of us who don't hold any firm religious convictions - and indeed, those of us who actively reject religion and the social engineering that it attempts - can still do good, can't we? After all, few things are more galling than a believer's insistence that theirs is the only way to live a life of meaning and kindness. Humanism can and does provide a framework for tolerance, compassion and respect of our fellow humans, no matter what their beliefs. So why would Roy Hattersley, writing in the Guardian (12/09/05), say that people with a religious faith are far more likely to get out in their communities and do good? Why would he conclude that "faith comes with a packet of moral imperatives that influence enough of them to make them morally superior to atheists like me"? He mentions a Salvation Army worker he knows who, despite condemning drinking, drug use and gambling, goes out on the streets every night to help people whose problems stem from just those practices. He also discussed how almost all the groups helping those in need after Hurricane Katrina had some kind of religious origin.

Hattersley is right to say that there are few explicitly humanist or atheist groups operating at the forefront of international relief efforts or, at home, helping to catch those who fall through society's cracks. And he's also right to say that faith-based groups are particularly successful in appealing to people's charitable instincts.

But do religious charities really dominate the charity industry? The Charities Aid Foundation (CAF) is a non-profit organisation that researches the charity and voluntary sector. Taking the international aid sector as an example, CAF's latest figures show that the top five fundraisers are Oxfam, Save The Children, the British Red Cross, ActionAid and Concern Worldwide. Although some of these organisations were originally faith-based in nature, today all are secular - and between them they get 59 per cent of the voluntary income of all the international charities.

But across the sector as a whole, faith-based charities are big business. Research released in December 2005 by CAF and the National Council for Voluntary Organisations found that faith-based organisations received 13 per cent of all donations to charity in 2004, attracting the highest average gift of £28.20. So-called 'high-level donors' - those who give £100 or more to charity each month - were more likely than the average giver to donate to religious charities. And that's not all: those charities will, through the Gift Aid scheme, be able to claim their donors' tax back on those gifts, and for donations of shares or property there is tax relief available to the donor as well.

Then there's the huge arena of charitable trusts: private bodies that make grants to charities. According to the Association of Charitable Foundations, these trusts make some £2 billion in grants, and nearly 10 per cent of these foundations give to causes related to religion.

When you look at the history of charity, it's little wonder that religion has played such a big part. The idea of doing 'good works' with the aim of improving one's chances in the afterlife dates back to ancient Egypt and beyond, and by the time Geoffrey Chaucer was writing Canterbury Tales, England's poor were playing a crucial role in society as recipients for the benevolent deeds of the wealthy, who hoped to reduce the time they'd spend in purgatory. The famous 'four heads of charity' - by which England's Charity Commission still determines whether an organisation qualifies as a charity or not - explicitly includes the advancement of religion as a charitable purpose. The new Charities Bill, which should come into force in early summer 2006, will change the law slightly - but it's the first real overhaul that England's charity laws have had in 400 years.

Along with the changes in society have come changes in the way that faith-based charities do business. Most of them have moved away from the Victorian notion that needy people should be forced to attend church services or read religious texts before they can benefit from charity. And the most successful faith-based charities have widened their focus to involve the public at large. Christian Aid and the Salvation Army consistently rank among the UK's top fundraising charities because they have such a broad base of support - people give to them not necessarily because of their particular religious belief but because of the wider principles they promote.

There are a minority of Christian charities which make clear decisions about supporting causes based on the tenets of faith. CAFOD, the Catholic Agency For Overseas Development, is explicit about its stance on abortion: it does not support it in any form, and joined other faith-based groups in lobbying to have the issue of 'reproductive health' removed from the UN's Millennium Development Goals. Most Islamic charities concentrate on projects in Muslim countries or within the British Muslim community, while Jewish charities have raised large sums for work in Israel. Cathy Pharaoh of CAF believes that, in contrast to the broader approaches of Christian Aid or the Salvation Army, some groups become successful because they stand up for their religion and its tenets.

Some charities place missionary activities as a key component of their work, and among them the Salvation Army is probably the best-known. The organisation is explicit about its evangelical efforts and its mission "to preach the gospel of Jesus Christ and to meet human needs in His name without discrimination." This deeply religious worldview doesn't seem to have a major impact on the Army's success as a charity; CAF ranks it as the UK's sixth-biggest fundraising charity, attracting some £90.9 million in donations in 2004.

On the whole, though, the biggest and most successful faith-based groups have got that way because they have soft-pedalled their approach to religion - choosing to treat it as an inspiration in their work, rather than a key aim of their organisation. Interestingly, one senior policy analyst for a Christian NGO told me that many devout Christians would be quite annoyed that his organisation isn't more explicitly Christian in its focus. His NGO doesn't incorporate prayers or religious texts into its work, many of its staff aren't Christian and they don't seek to discriminate between Christians and non-Christians in the work that they do.

So how do these organisations incorporate their religious beliefs into their work? If they don't actively seek to recruit new followers and they don't look to only help members of their own faith, then what role does religion play? And if being explicitly faith-based is a turn-off for people who don't share that faith, why do it at all?

According to one NGO worker I spoke to, faith provides these groups with a framework, a set of guiding principles. For most Christian charities, this centres around very broad aims of love for one's fellow man and the desire to seek justice for the oppressed and a fair deal for all human beings. But don't most people believe in those things? And if you agree about these broad aims, why should it matter what the motivation is? Why bring religion into it at all?

Firstly, religious affiliations provide these groups with a solid base for fundraising. Some religions go as far as to suggest a fixed portion of one's income should be donated to charity - the Church of England has recommended 5 per cent, while the Muslim concept of zakat suggests that followers give an amount equivalent to 2.5 per cent of their savings and valuables. To put it in its most basic terms, since people are required to give, religious charities can take advantage of that captive audience.

From the organisation's point of view, not only does faith provide a useful base for fundraising, but it gives a certain clarity to their aims and purposes. People I spoke to talked about the idea of moral certainty: the idea that religion gives them some kind of compass, a grounding for their beliefs that frees them from tricky arguments about relativism and definitions.

So why are there no big humanist charities, waving the flag at the forefront of international disasters? In the wake of Hurricane Katrina the American Humanist Association redoubled its call for a permanent, unambiguously humanist charity - since so many of the charities that were leading the relief effort were so clearly religious in their focus, it was felt that this would offer a clear alternative. In the UK, though, there's little demand for this kind of charity. Why not? According to Marilyn Mason of the British Humanist Association, it's because there are so many charities doing the things most humanists would want charities to do without any religious connotations at all. When the majority of Britain's charities are secular, there's little need to set up separate organisations.

So how should atheists or humanists decide who to give to? The answer here is pretty much the same as for any potential charity donor: work out what kind of causes are important to you, find out who's working in those areas and do some research.

There's little doubt that few humanists would feel comfortable giving to explicitly religious groups like the Salvation Army or Samaritan's Purse (who conduct an annual 'shoebox' appeal for children in Eastern Europe and have a strong evangelical aspect to their work). The same goes for organisations like Christian Aid, who may wear their faith a little more lightly, but who still consider it to be a major inspiration in their work. But the good news is there are lots of charities in every sector which have no religious focus at all. If you have any doubts, don't be afraid to pick up the phone and call the charity - they will be more than happy to talk through your concerns.

Humanists are generous with their time and money. A survey conducted by the BHA in 2000 showed that less than 1 per cent of respondents didn't support charity giving - and on average, the humanists surveyed regularly gave to six charities each. This compares pretty favourably to the general public; in the same year, just 36 per cent of Britons contributed to five or more charities.

So it would seem that Roy Hattersley's fears about moral inferiority are unfounded. Humanists and atheists do give, and they give a lot. It's just that, in absence of a clear label, their giving may not seem to be motivated by their religious beliefs (or, more correctly, the lack of them). They give to help other people, to supplement the work of the state, because in our current society it's the right thing to do.

In the context of the huge spending power wielded by faith-based charities - not to mention the generous charitable trusts and grant-making foundations - humanist and atheist concerns may be less visible. There are, as Marilyn Mason points out, no charities called Atheist Aid (at least, not yet). But secular charity is a powerful force. And if you're concerned about the size and influence of religious charities, there's one clear way of trying to redress the balance: get on the Internet, find out about some charities you do agree with, and give.

Jessica Williams new book How To Give To Charity is published by Icon Books