DOWNING STREET has made an informal complaint about Scotland Yard’s handling of the cash-for-peerages investigation.

The office of Sir Ian Blair, the Commissioner of the Metropolitan Police, received the complaint last week, according to Westminster sources.

Downing Street is threatening to upgrade the warning to a formal complaint if there is no change in the way the investigation is dealt with.

Reports at the weekend suggested that No 10 is furious over leaks about the investigation, which it is blaming on the police. Downing Street sources are reportedly suggesting that they believe that any trial would not now be fair.

Until now Sir Ian has excluded himself from the investigation because of his links with the Prime Minister. He has left Deputy Assistant Commissioner John Yates to take the lead.

The complaint will pose a dilemma for Scotland Yard because the Commissioner will not want to be seen to be meddling in the investigation, while ignoring the complaint altogether could provoke a row with Downing Street.

John Prescott said yesterday that he was completely happy with the investigation, dismissing talk of any complaint as speculation and “press prattle”.

But Lord MacKenzie of Framwellgate, the Labour peer and former head of the Police Superintendents’ Association, warned the Prime Minister not to try to wreck the inquiry. “The common ploy by criminals is to get their retaliation in first and make a complaint against the police to divert the attention of the authorities and put the heat on the police,” he told The Mail on Sunday. “But it rarely works. If Downing Street is considering attempting to do this, they are making a catastrophic mistake. The police have to do their duty.”

The complaint comes amid allegations that the Labour Party concealed loans from its own auditors by failing to mention them in its 2004 accounts. It has emerged that the party did not inform the auditors that it had received the loans until spring 2006 — a year after they arrived in the party’s coffers. This means that the auditors signed off Labour’s 2004 accounts in June 2005 not knowing that it had accumulated £12 million in loans the month before.

Police are considering whether there was a breach of the terms of the Political Parties Elections and Referendums Act 2000 (PPERA).

A leading accountant said: “The accounts that were signed off in December 2004 should declare post balance sheet events if they are so significant that it would be misleading not to. The £12 million loan was made after the balance sheet date and that is a significant figure. It’s a matter of judgment — but most people would agree they [the accounts] are misleading.”

Meanwhile, there was further pressure on the Prime Minister after notes appeared of a meeting with Labour’s National Executive Committee in March, in which he said that the party’s financial backers were “trashed in the media and so potential donors preferred the confidentiality of a loan”.

Any suggestion that loans rather than donations were being sought to deliberately conceal the donor’s identity could also breach the PPERA.

Sir Gulam Noon, who lent Labour £250,000 after previously making several donations, disputed the Prime Minister’s account. He said that Lord Levy, the Prime Minister’s chief fundraiser, wanted the secrecy. “I was very happy to contribute as a donation but . . . I was asked to give a loan,” Sir Gulam said

• A Labour MSP who is retiring at next year’s Holyrood election claimed yesterday that he had already been offered a peerage by Tony Blair (Angus Macleod writes).

  John Home Robertson, who is standing down as MSP for East Lothian, told The Sunday Times (Scotland) that the offer had been made in recognition of his services on a committee overseeing the completion of the controversial Scottish Parliament building in Edinburgh.

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