Tony Blair has been interviewed for a third time by Scotland Yard detectives investigating the cash for honours allegations, it emerged last night.

Sources confirmed that the former Prime Minister, who left office on Wednesday, had once again been questioned by officers, possibly within the past week. It is thought he was questioned as a witness and not as a suspect.

However, the cross-examination was once again kept secret in what appeared to have been an attempt to ensure the news broke once Mr Blair had left office.

On Monday, his spokesman made clear the ex-PM had not been questioned again. When asked if there had been any contact with the police, he said his answer of recent weeks - ie that there had been no contact - remained unchanged.

It is believed the spokesman had not been informed of the third interview. When Mr Blair was questioned the second time in January, his spokesman was kept out of the loop until after the event. At the time, the Metropolitan Police requested a six-day news blackout so they could interview other witnesses.

On the occasion of the former premier's first interview last December, his spokesman was asked if there had been any contact with the police, to which he said no. However, at that very moment Mr Blair was being quizzed by detectives at his Downing Street office. Again, it is thought that at the time the spokesman had been kept in the dark.

Since March 2006, police have been investigating claims - denied by the former Prime Minister - that four businessmen were nominated for peerages in return for making loans totalling more than £5m in the run-up to the 2005 General Election. However, it is thought that the probe has gradually moved from looking initially at these claims to one of an alleged cover-up.

So far, close Blair aides Lord Levy and Ruth Turner along with Labour donor Sir Christopher Evans have been arrested and released on bail. However, thus far there have been no charges. About 140 people are thought to have been questioned under caution or as witnesses. All deny wrongdoing.

A file was passed to England's Crown Prosecution Service (CPS) by the Met's Specialist Crime Directorate in April but no decision has been made on prosecutions. One is expected before the autumn.

Earlier this month, following a Freedom of Information request, Scotland Yard disclosed that the bill for the 14-month inquiry had been £773,177 between its launch on March 27, 2006, and April 30 this year. This is now likely to have increased significantly.

Last night, Angus MacNeil, SNP MP for Na h-Eileanan an Iar whose complaint to the police triggered the Scotland Yard investigation, said: "This new revelation comes as no surprise. However, the fact it has been kept secret until Blair left office is intriguing.

Claims that the police were issued with a threat from No 10 that Blair would have to resign if he was reinterviewed add to increasing evidence No 10 has put undue pressure on the investigating team by raising the prospect of a constitutional crisis."