Tony Blair has been interviewed for a second time by police investigating allegations of the sale of honours, it was disclosed today.
Scotland Yard said the Prime Minister was questioned in 10 Downing Street last Friday "to clarify points emerging from the ongoing investigation".
Police declined to say why they had placed a news blackout on the interview, which came a week after the January 19 arrest of Mr Blair's close aide Ruth Turner and days before his fundraiser Lord Levy was arrested on Tuesday this week.
Mr Blair was questioned as a witness, not a suspect, and was not placed under caution for the interview, which lasted less than one hour.
He was accompanied by a Downing Street note-taker, but did not have a lawyer with him, said his official spokesman.
In a statement, the Metropolitan Police said that Mr Blair "co-operated fully" with detectives.
Mr Blair became the first sitting Prime Minister to be questioned as part of a criminal investigation when he spoke to detectives for two hours on December 14, in what was then thought to be the climax of an inquiry launched in March last year.
But subsequent weeks have seen the police team, headed by Assistant Commissioner John Yates, broaden their inquiries beyond cash-for-honours allegations to look into the possibility of a conspiracy to pervert the course of justice.
Neither Scotland Yard nor Downing Street would today discuss in any detail the contents of Mr Blair's second interview.
The Shadow Chancellor George Osborne said he did not want to "prejudge" the police inquiry.
"But it's obviously an extremely serious matter when a serving Prime Minister is interviewed twice by the police," he added.
"I think that the police need to conduct the inquiry as they see fit and it's certainly not the job of politicians to interfere in that."
He told Sky News: "There's a broader point here which goes to the whole situation we find ourselves in this year, which is that this Government is paralysed and, as David Cameron was saying in the House of Commons yesterday, it's difficult to see why Tony Blair is remaining in office."
Arriving at the Royal Surrey County Hospital in Guildford, where he was touring the Accident and Emergency unit, the Conservative leader Mr Cameron said: "It is obviously a serious matter when a serving Prime Minister is interviewed twice by police but I think we have to await the outcome of the inquiry."
It is not known whether detectives from the Metropolitan Police's Specialist Crime Directorate will speak to Mr Blair a third time, with his official spokesman saying only that it was "a matter entirely for them".
The spokesman said news of Friday's interview was such a closely guarded secret that even he and other members of the Number 10 press office were not told until yesterday afternoon, when police said it could be made public.
At daily press briefings over the past week, the spokesman was repeatedly asked whether the Prime Minister had spoken to police or been approached for a further interview, and on each occasion said he was not aware of any such development.
But he today denied misleading the media, pointing out that he was careful to say only: "As far as I am concerned, nothing has changed." The press was informed "at the first appropriate moment", he said.
The spokesman declined to say whether anyone else in Downing Street was told about the development, saying only that the information was "kept extremely tight".
Although no charges have yet been brought in the cash-for-honours investigation, news of Mr Blair's second interview, coming hot on the heels of the arrests of two of his closest allies, will heighten pressure on him.
The investigation - launched after Scotland Yard received complaints that wealthy individuals who lent money to bankroll Labour's 2005 General Election campaign were later nominated for honours - has now lasted more than 10 months, led to four arrests and seen some of Mr Blair's closest allies questioned by detectives.
SNP MP Angus MacNeil, whose initial complaint sparked the police inquiry, said today's news marked "another bleak first in British politics".
"Mr Blair's situation is looking worse with every passing day. He appears to be in very deep trouble," he said.
"He's questioned and then a few days later his chief fundraiser Lord Levy is re-arrested. I'd say this marks another escalation in the police inquiry."
Mr MacNeil said that Scotland Yard's request for a news blackout "gave the lie" to suggestions, apparently emanating from Downing Street, that police were leaking details of the inquiry.
Senior figures at the Metropolitan Police Authority have alleged there is a Westminster "whispering campaign" against Mr Yates.
But Mr Blair's spokesman denied that he had ever pointed the finger at police over leaks, telling reporters: "We've all seen speculation in the papers... I presume it's coming from somewhere. I do not presume to say where from."
Friday's interview came as Downing Street was fending off claims that detectives had discovered a hidden computer network in Number 10 from which crucial emails appeared to have been deleted.
The previous evening ITV News had reported that the existence of a second IT system in the building was disclosed to police by a witness over recent weeks - possibly inadvertently.
The information was said to have led to the arrest of Ms Turner on suspicion of perverting the course of justice.
The ITV report, dismissed by No 10 as "untrue", also claimed officers discovered emails last autumn which were apparently exchanged between Lord Levy, Ms Turner and Mr Blair's chief of staff, Jonathan Powell.
They were said to be "indiscreet" and referred directly to "Ks and Ps" - knighthoods and peerages - being offered to those who have donated money.
Friday also saw Mr Blair deeply embroiled in the crisis at the Home Office. He was forced to defend John Reid in a TV interview, after a second judge said he was releasing a sex offender because of the Home Secretary's guidance over the shortage of jail places.
Shortly after talking to police, Mr Blair - who also spoke by phone with German Chancellor Angela Merkel that morning - flew to Switzerland to attend the World Economic Forum in Davos.
He appeared relaxed at meetings in the ski resort, including talks with Brazilian President Lula da Silva, South African President Thabo Mbeki, U2 singer Bono and Microsoft chairman Bill Gates. He later attended a party thrown by Bono, also attended by media tycoon Rupert Murdoch.
Mr Blair's spokesman today insisted the PM was not being distracted from his job by the investigation.
"The Prime Minister gets on with the business of government," he said.
"If you look back at the last 12 months on the issues that have come to the fore and are still being driven forward, you can see that this is a government which is moving forward with a very, very strong agenda."
Asked whether Mr Blair was eager to see the lengthy investigation brought to a conclusion, the spokesman said: "What the Prime Minister is determined is that No 10 will fully co-operate with the police and they will carry out their investigation as they should."
He added: "There is a police investigation, no one should prejudge the outcome of that investigation in any way. The important thing is that the Prime Minister is getting on with the business of government."
Plaid Cymru parliamentary leader Elfyn Llwyd, who also made a complaint to police last March, said: "It's inevitable now that the police will concentrate all of their efforts on Number 10.
"I can't believe under any circumstances that Lord Levy would have been acting in the manner alleged without the tacit understanding of the Prime Minister in that, bearing in mind the close proximity of each man to the other."
The Liberal Democrat leader Sir Menzies Campbell's chief of staff, Edward Davey said: "It is clear that this inquiry is going to haunt Tony Blair throughout his last months in office and beyond.
"What is important at this point is that Labour MPs stop sniping at the police from the sidelines and allow them to continue to do their job professionally."
Earlier this week, Mr Davey claimed there was "a whiff of Watergate" developing around the cash-for-honours affair.
But Labour MP Clive Betts played down the seriousness of the latest development.
"Let's be clear, he is being questioned as a potential witness, he is not under threat of a charge," he told Sky News.
He urged the police to come to their conclusions quickly and present any evidence to support the allegations.
Conservative backbencher Nigel Evans said the police should be allowed to take time if that was necessary.
But he added: "The fact is the public need to be kept informed about what is going on."
Scotland Yard ought to clarify what officers asked of Downing Street and the Prime Minister: "This has clearly very serious implications. We are now talking about did Downing Street get involved in a cover-up about whether the Prime Minister had a second interview."Reuse content