TONY Blair became the first prime minister in history to be questioned as part of a corruption investigation when the cash-for-honours inquiry finally reached No 10 yesterday.

Scotland Yard detectives questioned Mr Blair for about 90 minutes about his nomination of four Labour supporters to the House of Lords. It later emerged they had loaned the party £4.5m.

Mr Blair was not questioned under caution and was not accompanied by a lawyer.

While that suggests the Prime Minister is not a suspect in the inquiry, it does not rule out further questioning - on different terms - in the new year.

The bare fact that detectives visited Downing Street is hugely damaging to Mr Blair. Past prime ministers have only been questioned as part of leak inquiries.

There were immediate suspicions that No 10 had sought to "bury the bad news" by arranging the interview on the day attention was focused on the report into Princess Diana's death.

Furthermore, within hours, Mr Blair had left Britain for a Brussels summit and a lengthy trip to the Middle East.

But the Prime Minister's spokesman denied the interview was cynically timed. He said: "Categorically, that was not a factor at all.

"There was no link to other events taking place."

The spokesman refused to say whether No 10 or the police had arranged the interview.

Asked if he expected Mr Blair to be questioned again, he said there had been "no indication" to suggest he would be.

Angus MacNeil, the Scottish National Party MP whose complaint triggered the police inquiry, said: "This revelation will be shaking the very foundations of Westminster.

"For the Prime Minister to be questioned by the police during a criminal investigation is unprecedented."

It emerged earlier this year that Labour secretly borrowed £14m for its General Election campaign fund. Four of those who lent money were later nominated for peerages.

A corruption investigation was launched into possible breaches of the 1925 Honours Act and the 2000 Political Parties Act.

The inquiry started closing in on Mr Blair in July, when Lord Levy - Labour's chief fundraiser and the Prime Minister's tennis partner - was arrested and interviewed.

Since then, senior Labour figures questioned as witnesses include Darlington MP and former Cabinet minister Alan Milburn and Health Secretary Patricia Hewitt.