Does Tony's new best friend know something we don't?

She's the Blairs' woman who "does". When Tony and Cherie need to throw a party, set up a lecture tour or even find a house they turn to Martha Greene.

Now the 50-year-old US-born socialite and fixer has quietly carried out another errand for the Blairs, one that provides a fascinating insight into their plans for life after No 10.

On 4 November the website domain blairfoundation.org.uk was registered - by one Martha Greene. It is the clearest sign yet that the Prime Minister plans to set up a body similar to that established by the former US President Bill Clinton.

Ms Greene, who is said to want to be the Blairs' manager after they leave Downing Street, may have registered the website on her own initiative. But Mr Blair dropped a hint that he was considering an ambitious "global initiative" just before Christmas. "I think the single thing for me that is most important is that whatever I do afterwards has a real purpose to it, that it is not just about doing a job.

"This is a position that, once you have occupied it, you have done something that has what I call a real life purpose to it. And certainly in anything I wanted to do afterwards it would be a different purpose, but similar in its motivation.''

Motivation is something that the redoubtable Ms Greene knows all about. She came to Britain as a 25-year-old in 1981 and got a job as a secretary at the Saatchi & Saatchi advertising agency. She worked her way to the top of the firm, leaving in 1990 to set up an independent film production company with a colleague.

Eight years later she became a co-owner of Villandry, a trendy London restaurant in Great Portland street, a favourite haunt of New Labour's media elite.

Ms Greene shared her customers' social milieu: she lives in a £1.5m house in Islington just a few streets from where the Blairs resided before the 1997 election.

By 2000 her circle of friends included Carole Caplin, who had become Cherie Blair's lifestyle guru. She introduced Martha Greene to her famous client. Speaking about the meeting, the American self-made businesswoman said: " I trusted [Cherie] immediately, which I don't often do with people... She's got a strong personality ... she's passionate, which I find really admirable. "

It seems it was an attraction of the like-minded. Jeff Stark, a former business partner, describes Ms Greene as "straight to the point, no bullshit; people either love her or hate her".

Ms Greene catered for the Blairs' 20th wedding anniversary party and by 2002 had moved from the kitchen to the table at a private birthday dinner at Claridges for the Prime Minister.

Martha Greene finally supplanted Carole Caplin as Cherie's principal confidante in the wake of the Blairs' disastrous purchase of two flats in Bristol.

The deal, in 2002, was brokered by Ms Caplin's then boyfriend, Peter Foster, a convicted conman, a fact initially denied by the No 10 press office acting on the instructions of Cherie Blair.

But Ms Greene appeared to have slipped herself when helping the Blairs secure the third property in their portfolio. The Prime Minister and his wife were widely thought to have paid substantially over the market price in paying £3.6m for a town house in Connaught Square in central London in 2004.

Further controversy followed when it emerged that she had helped set up a lunch between Jonathan Metliss, her current partner and the owner of a health firm that bids for NHS contracts, and Professor Paul Corrigan, the Prime Minister's health adviser.

But it seems that Ms Greene, who helped find the house and acted as a go-between in its purchase, has been vindicated in that capacity at least by London's rising house market.

Jeremy Karpel, of the up-market estate agent T K International, estimates that a flood of money, fuelled by generous City bonuses, has helped push the value of the Blairs' home to around £4m.

"They probably paid around £200,000 more than they should have done, but they are well in profit now," said the agent, who is selling a neighbouring property on the square for around £4.25m.

But when will the couple finally move in - and what will they do with themselves once relieved of the pressures of office? Here, too, it seems Martha Greene has been busy. She has accompanied Cherie Blair on a number of lecture tours which she helped set up. She is said to want to manage the Blairs' speaking commitments full-time in the future.

And now she is making even more ambitious preparations for the future. The registration of blairfoundation.org.uk came after two previous versions, blairfoundation.co.uk and blairfoundation.org, had been snapped up.

Gavin Wiseman, a 30-year-old online gambling entrepreneur, admits he registered the latter domain as a speculative venture.

"I was just amazed to see nobody had thought of it. I wasn't really doing it for money - I just wanted to see whether I'd get a phone call from No 10."

But it seems that the capable American, the superwoman who "does", had a better idea. She simply registered another domain for the putative charitable foundation that will provide Tony Blair with a political identity from this summer.

Contacted by The Independent on Sunday, Ms Greene said: "I really don't want to talk to you."

Recently Mrs Blair has also grown increasingly close to Dwina Murphy-Gibb, a part-time Druid priestess and artist who is married to the Bee Gees singer Robin Gibb. The Blairs have recently returned from a holiday at their mansion in Florida. During the holiday the Prime Minister is reported to have been treated to a four-hour fishing trip on a yacht owned by Justin Williams, a leading Bermuda-based lawyer.