Cherie must choose between her legal career or lecture tours

Lady Butler-Sloss, who was Britain's most senior woman judge, said the Prime Minister's wife had the potential to become a successful judge.

But she warned such a promotion was unlikely if Mrs Blair continued to pursue her media interests and lecture work.

If she wanted to become a judge, she would have to put in two years of uninterrupted practice at the Bar combined with sitting as a part-time recorder.

Lady Butler-Sloss told the Daily Telegraph: "Is her heart in the Bar and the prospect - which is never certain - of becoming a judge? Or would she prefer a different way of life?

"It might be partly a question of money - but also which she would regard as more rewarding. She has a lot more to give to family law. She has a genuine and very strong commitment to the welfare of children and I would hope that she would continue to promote that. I much admire her for it."

Lady Butler-Sloss, who is stepping down as the coroner at the inquest into the death of Diana, Princess of Wales, said she wanted to rebalance perceptions about Mrs Blair. "She is a very warm and loving person," she said.

Her advice came as Mrs Blair prepares to embark on a lecture tour of the United States and Canada and re-issue her memoirs.

The Prime Minister's wife is being billed as "Cherie Booth QC. Human rights lawyer, wife of Tony Blair."

All three venues, with a capacity of up to 3,000, are said to have been sold out.

In another sign that her long-term interests may lie outside the judiciary, Mrs Blair has granted the BBC unprecedented access for a documentary on her last months in Downing Street.

Mrs Blair has allowed a crew, led by presenter Fiona Bruce, to follow her at home and across the world on public engagements over four months for the fly-on-the-wall programme, which has cost an estimated £100,000.

Her biographer Linda McDougall said she was not surprised that Mrs Blair had decided to take part in the documentary.

"This will help to keep up her profile," she said.

A spokesman for the BBC said Mrs Blair had not been paid and there was no date for when it will be broadcast.

He said: "This is a programme which is still in production. There is no date yet for when it will go out, but it will probably be shown on BBC2."

Downing Street declined to comment.