This week, the news cameras also went into action as Victoria Adams - Posh Spice - launched a new campaign by the Meningitis Research Foundation, while the Duchess of York donned special sunglasses to publicise how to watch this summer's eclipse in safety.

Whenever a disaster appeal is launched or a charity begins a new campaign, there is one thing you can be sure of - there will always be a celebrity.

The Duchess of York supporting a cancer charity ball in New York

No-one doubts the sincerity of superstars who lend their names to worthy causes, and many of them have backed other campaigns. In most cases, they genuinely believe in what they are supporting.

But one can be forgiven for wondering how long the star sticks around after the photocall is over, how much commitment is behind their support.

So who are the celebrities who care enough to show real dedication and help out when there's no prospect of a photograph in the papers? There are some - and in many cases, their unsung dedication goes on for years.

Work unnoticed

Few showbusiness stars can match what Princess Diana achieved in making secret night-time visits to comfort the sick and homeless.

But one famous name who comes close is Lord Rix - the former actor Brian Rix - who travels the country regularly visiting homes for people with learning disabilities as president of Mencap.

Lord Rix works hard behind the scenes for Mencap

He has been involved with the charity for more than 40 years, since his own daughter was born with Downs Syndrome, and now helps plan the charity's campaigns. He also raises disability issues in the Lords, where much of his work goes unnoticed by the public.

"He does an awful lot because he feels so strongly about the issues," said Mencap spokeswoman Alison Jones.

Norma Major is another of the charity's patrons, giving up her time - away from the spotlight - to serve on its Challenge Fund committee and helping her local Mencap group.

Feel-good factor

Newsreader Alistair Stewart regularly makes time in a busy schedule to lend his support to a handful of charities including NCH Action for Children.

He does it because he passionately cares about the cause and if he can help, so much the better.

He said: "It really is no exaggeration to say that each day's postbag brings some request from one charity or another. To avoid spreading one's time too thinly and benefiting no-one, choices have to be made.

"NCH Action for Children deals with issues I care about in a totally professional way...

Joanna Lumley is dedicated to CIWF

"Charity to me means doing a little good and paying back a bit. Working with professionals who do the real work and willingly adding that little extra they know we can provide. It is a genuine 'feel-good' factor on all sides."

The charity also regularly has help from actresses Jenny Agutter and Shirley Ann Field, star of My Beautiful Launderette, who are long-time supporters.

No glamour

Actresses Joanna Lumley and Sue Jameson also staunchly support their favourite charity, Compassion In World Farming, without the glamour of publicity.

Ms Jameson - who starred in Catherine Cookson's The Girl - told BBC News Online she gives her backing regardless of having a famous face - but is also happy to play on her status if it helps.

"There's no publicity in it for me," she said. "I first got involved in demonstrations against live exports at Shoreham. I've done street collections, which is soul-destroying, and now I'm a trustee.

Penelope Keith: "You can't compartmentalise caring"

"It has nothing to do with being a celebrity - I do these things because I believe in them passionately - but I'm delighted to use my status for CIWF if I can."

Former racing commentator Sir Peter O'Sullevan and actress Penelope Keith also lend their support when there is no sign of a camera.

Ms Keith said: "I care for lots and lots of things. You can't compartmentalise caring. If you're a caring person you care for most things and animals need an awful lot of that too."

Superstar ambassadors

Unicef too has its long-time superstar supporters. Some are so devoted they are given the title Special Representative or Goodwill Ambassador.

Danny Kay was the first; he recruited Audrey Hepburn. Vanessa Redgrave, Roger Moore, Sir Peter Ustinov and Lord Attenborough have followed in their footsteps at international level.

Vanessa Redgrave is a Unicef ambassador

Singer Ian Dury and Lord Deedes are UK ambassadors.

"We're very careful in how we use celebrities - we can't just have someone who's famous for five minutes," says a spokeswoman.

She said some stars are just as dedicated but don't have much time to spare - Robbie Williams, who last year joined a Unicef campaign in Sri Lanka, for instance, but is unable to have time to be an ambassador.

Nor is the work glamorous. Vanessa Redgrave has put up with long, hot, bumpy rides through dangerous territory, staying in uncomfortable conditions, with no accompanying photographers, to support Unicef.