Blair was interviewed last Friday as a witness in the investigation into alleged illicit financing of his ruling Labour Party, rather than as a suspect, it said in a statement.

"Last Friday the prime minister was briefly interviewed by the police as a witness," said Downing Street, adding that the questioning -- which lasted less than an hour -- was kept secret at the request of police.

The announcement came just two days after Lord Michael Levy, his Middle East envoy and chief fundraiser, was arrested for a second time in the probe.

Two weeks ago, another key figure close to Blair, Ruth Turner, director of government relations at his Downing Street office, was also arrested.

Levy was arrested on suspicion of conspiracy to pervert the course of justice before being released on bail, while Turner was held on suspicion of breaching the Honours Act 1925 and perverting the course of justice.

Their arrests have prompted speculation that the police investigation is snowballing. Officers investigating the case requested a news blackout on the interview, which was only lifted Thursday.

The so-called "cash-for-honours" probe was launched last March and has seen Blair and his entire cabinet interviewed by detectives.

It is trying to establish if his Labour party and other parties illegally offered seats in Britain's unelected upper chamber of parliament, the House of Lords, in return for financial assistance.