Cherie sits in plush hotel waiting for a far more profitable collection

When Tony Blair finally returns to the British embassy in Washington on Tuesday after a joint press conference with George Bush, exhausted by an overnight flight and a demanding round of meetings, he will find his wife waiting for him, refreshed by a comfortable night in a plush hotel.

When Tony Blair finally returns to the British embassy in Washington on Tuesday after a joint press conference with George Bush, exhausted by an overnight flight and a demanding round of meetings, he will find his wife waiting for him, refreshed by a comfortable night in a plush hotel.

The "First Lady of Downing Street" - as she is billed - will address an audience of up to 2,500 on Monday night at the Kennedy Center in a speech organised by the Center for Association Leadership, a Washington think tank that provides training courses for not-for-profit organisations. Her fee is believed to be in the £20,000 -£30,000 range.

The coincidence that both Blairs were in the American capital on the same day has produced a storm of criticism, including a call from the Conservatives for an investigation by the Committee on Standards in Public Life.

Under US law, the First Lady is barred from making any profit from her role as wife of the President, and proceeds from such an engagement would have had to be donated to charity. But there are no equivalent rules in the UK on governing how far a Prime Minister's spouse can use her other half's public position for her own private profit.

This is principally because no previous Prime Minister has been married to someone with so active a public life as Cherie Blair, who has written for the "Sunday Review" section of this week's Independent on Sunday on why she is still a practising Roman Catholic.

In addition to being an eminent barrister, Mrs Blair's semi-public status has enabled her to earn money as a writer and lecturer, but has also exposed her words, habits, friendships and her attempts to enter the property market to relentless public exposure.

Downing Street has pointed out that her Washington visit was arranged months ago, before she knew that Mr Blair would be in the same city in the same week, and that she is both flying out and returning separately. But officials also point out that when the Prime Minister is on an official visit to any foreign capital, and his wife is with him, normal practice is for them both to be put up and looked after by the British embassy. But any hospitality that Cherie Blair accepts from British diplomats on this occasion is liable to lead to accusations that she has made a private profit at public expense.

However, as a well-known author, Cherie Blair appears to suffer from being more read about than read. Her much-hyped book The Goldfish Bowl, about the lives of prime ministers' wives, is being offered at a cut price by Amazon, the internet bookseller, on whose top seller list it ranks at number 339,933.