`I really care about... what was the cause again?'

"It's a bit like going into a boxing ring - there's nowhere to hide ... Or like being in a cage. People are prodding you, asking you for autographs but it's part of the deal," says Rory Bremner.

No charity can compete in a pressured world now without celebrity endorsement - but the biggest showbiz personalities are receiving in excess of 50 requests a week, according to agents.

Even Camilla Parker Bowles has taken the plunge - becoming a patron for the National Osteoporosis Society, and Thursday's Countryside March saw Jeremy Irons and Paula Hamilton rally to the cause.

The result can be huge amounts of free publicity - the Health Education Authority (HEA) estimates it received "tens of thousands of pounds worth" for their nutrition drive when they secured 30 radio interviews for Ken Hom, the chef, at Chinese New Year.

But when times are hard the competition is so severe that celebrities are having to cut back on what they do. Otherwise no sooner have they unbuckled the pads at the cricket match than it's on with the stand-up comedy and over to the barbecue. Or such was the case this week at the Allan Lamb and John McCarthy cricket match in the grounds of Althorp house, in aid of the Medical Foundation for the Care of Victims of Torture, and the Cystic Fibrosis Holiday Fund for Social Workers.

"We're all becoming social workers. Everything is coming down to marketing. If you don't have proper marketing people don't take charities seriously. People don't listen unless there is a celebrity there," says comedian Rory Bremner, one of the celebrity cricketers.

"I wouldn't describe myself as Mr Charity," added Ian Hislop, who was also attending, at the invitation of John McCarthy.

"I could do nothing else all year but I limit myself to things that mean something to me.

"I hate people who turn up to lots of benefits saying `we really really care - what was the cause again?' With things like that having celebrities can be counterproductive."

He prefers to help a leukaemia charity and a hospice, both of which have personal significance, but often fail to grab the attention the more "glamorous" charities do.

"We do live in an age where charities have to function increasingly as businesses and the value in terms of news coverage and awareness of using a celebrity is obvious," said Helen Fielding, who has produced documentaries for Comic Relief and is author of Cause Celeb, a novel about the celebrity- charity link.

"It is very important that celebrities should be well informed, responsible and effective messengers but there's no reason to get a downer on them per se."

Charities must learn to pick their celebrities carefully, agreed Jenny Hay, head of press for the HEA. People for the Ethical Treatment of Animals were also less than amused when Naomi Campbell, who had previously declared "I'd Rather Go Naked Than Wear Fur" for their campaign, strutted down a Milan catwalk wearing animal skins.

The publicity agent Max Clifford advises charities to secure Diana, Princess of Wales as the sure-fire way to get column inches, or failing that Cherie Booth, wife of the Prime Minister, whose profile is also good at the moment. Otherwise, charities should be lobbying George Clooney, Oasis or the Spice Girls.

Both the Prince of Wales and Camilla Parker Bowles are not popular enough to draw in the crowds, although the worst possible situation would be "Jonathan Aitken heading a morals campaign or John Major lobbying for strong leadership".

To be fair, celebrities could argue they need to pick their good causes just as carefully, after Paula Hamilton found out this week on the Countryside March. She ended up being bitten by the ferret she was posing with.