Detectives have discovered a hand-written note from Tony Blair among new evidence that has widened significantly the cash-for-honours investigation.

It is the first time that the "paper trail" uncovered by Scotland Yard has led directly to the Prime Minister. The note is understood to acknowledge the efforts of Labour's 12 secret lenders who provided £14 million to help the party fight the 2005 election.

The Prime Ministers's comments were among a batch of Downing Street papers obtained by detectives. He had written in ink on typed, internal government papers and initialled his views.

Officers now believe that Downing Street intended to give working peerages to most of the lenders. Those in line included Sir Christopher Evans, the multi-millionaire entrepreneur, who is the only lender arrested as part of the 10-month inquiry.

The disclosures are deeply embarrassing for Mr Blair, whose problems escalated when Ruth Turner, one of his closest aides, was arrested nine days ago. Labour officials and the lenders deny acting illegally, although the loans were so secret that not even Jack Dromey, the party treasurer, knew about them.

Until now, it was believed that only four of the 12 lenders had been in line for peerages. But detectives are convinced that at one point the Prime Minister intended to put forward Sir Christopher, along with - among others - Sir Gulam Noon, Sir David Garrard, Dr Chai Patel and Barry Townsley.

Senior sources said, however, that Mr Blair was advised against this. Detectives believe this may have been linked to Merlin Biosciences, one of Sir Christopher's companies, being the subject of a Serious Fraud Office inquiry.

Scotland Yard also suspects that Mr Blair was reined back from recommending more than four peerages at the same time because party officials believed it was prudent to stagger them in case - as happened - the loans became public knowledge. Elfyn Llwyd, the Plaid Cymru leader and one of two MPs whose complaints sparked the police investigation, said: "This shows that the Prime Minister has many questions still to answer. I remain absolutely convinced that police will need to interview Tony Blair again."

Mr Blair's recommendations of peerages for the four lenders went to the House of Lords Appointment Commission in late 2005 and were blocked.

Some of the 12 secret lenders were not eligible for a peerage: Lord Sainsbury, who lent £2 million, had been created a life peer in 1997. Mr Blair also personally opposed giving a peerage to one of the lenders, according to sources.

Detectives are preparing a dossier that seeks to prove criminal intent: that at least some of the loans were linked directly to the promise of a peerage. Senior ministers privately acknowledge that the affair is one of the main reasons for the erosion of Mr Blair's political power.

One Cabinet minister said that he agreed with those Labour MPs who want the Prime Minister to step down before his preferred exit date of June if any senior Downing Street figure is charged. "This is absolutely hammering us as a party," the minister added.

The Sunday Telegraph revealed last week that detectives had to "hack" into Downing Street computers in their search for evidence. That came after officers became convinced that emails and other evidence had been withheld amid suspicions of a "cover-up".

Ruth Turner, Sir Christopher and Lord Levy, Labour's senior fundraiser, remain at the heart of the police investigation: all three deny acting illegally. At first, the police concentrated on whether honours had been illegally promised in return for loans, and whether the loans had been at commercial rates. They are now also investigating whether officials perverted the course of justice.

Detectives do not necessarily think there is sufficient evidence to bring charges against the Prime Minister. But he is increasingly likely to be called as a witness if charges are brought against senior aides.

Last night, No 10 denied the existence of an incriminating document written by the Prime Minister.