CONTEXT: Who are they?They are your worst nightmare, barging into your life accompanied by a camera crew and a list of unreasonable demands. There you are, minding your own business, getting on with running the country, when suddenly they set up camp on your doorstep, and begin demanding money with menaces. They won't leave you alone until you do their bidding, and if you refuse them, they will denounce you in front of millions of TV viewers, film fans and concertgoers. They are the celebrity lobbyists, and they are running amok in the corridors of political power.

Call security!Alas, once a celobbyist has set their sights on you, there's little else you can do except roll over and play dead. Like a rabbit in the headlights, once you are caught in the piercing glare of their enormous fame, you cannot run and you cannot hide. Might as well smile for the world's media, give the celobbyist what they want, and hope they go away.

How awful of those celebrities to stalk and harass those poor politicians. How would they like it if it was done to them?The latest victim was UK immigration minister, Phil Woolas, who incurred the wrath of actor Joanna Lumley, who has been campaigning for residency rights for Gurkha veterans. In an extraordinary live TV broadcast, the hapless Woolas was soundly handbagged by the Ab Fab star, after it emerged that, despite Home Office assurances, five former Gurkhas had their residency applications turned down. Woolas buckled under Lumley's cold, angry glare, and found himself agreeing to numerous concessions, including allowing lawyers for the Gurkhas to help draw up new guidelines for residency applications.

Well, when you've got Patsy from Ab Fab going "go on, sweetie darling, do it," how can you refuse?Politicians of all hues are finding it hard to "just say no" when the celobbyists knock on their doors. When Brad Pitt paid a visit to Capitol Hill to tout his green homebuilding project for victims of Hurricane Katrina, house speaker Nancy Pelosi was only too happy to receive him. For the clearly starstruck Pelosi, the real issue was not about housing 150 families and doing right by the people of New Orleans – it was about being able to brag to her children that she'd met Brad Pitt.

Isn't his missus a celebrity campaigner, too?Angelina has been a very busy mom, campaigning for refugees, and travelling the world in her role as a UN's goodwill ambassador. World leaders, dictators and despots roll out the red carpet when they hear Angelina approaching, because they know that their armies and juntas are no defence against the pouting lips of the Tomb Raider star. Besides, she might end up adopting a child while she's visiting, which might be good for tourism.

Why do they do it? Haven't they got movies to make and albums to promote?For many actors and pop stars, reaching the pinnacle of their craft is often not enough – they need something more to boost their self-worth, and what better way to boost your ego than to stand beside Barack Obama and tell him what to do? These days, no self-respecting star is without a pet cause – it's an essential accessory. George Clooney, Wyclef Jean, Richard Gere, Nicole Kidman – the parade of politicised pop and film stars grows bigger every day.

It's all Bono's fault, isn't it?He started it. The U2 star was not the first celebrity lobbyist, but he has always been the most adept at using his fame to highlight issues and gain the ear of politicians and policy-makers. Bob Geldof was so good at it, he ceased to be famous for anything else. Early celobbyists included Breakfast at Tiffany's star Audrey Hepburn, who, along with Danny Kaye, became goodwill ambassadors for Unicef in the 1950s.

Sounds like celebs should be running the world – they'd be so busy saving the world they wouldn't have time to make crap albums and rubbish movies.Not everyone is enamoured with the celebrity lobbyist. Some believe that actors and pop stars shouldn't be dabbling in issues they only have a rudimentary grasp of. They believe that celobbyists, with their quick soundbites and skill at playing the media game, are trivialising real humanitarian issues, and making things tougher for the nobodies who work at the coalface without getting the recognition. When Geri Halliwell aka Ginger Spice, became a UN goodwill ambassador, the PR damage to the organisation was incalculable.

Try at work: Hi hon, I'm tied up a bit here in Darfur – can you pick up a pint of milk on your way back from Cambodia?

Try at home:Er, Mr President, Air Force One is ready. Hurry before Ozzy Osbourne gets here.