The newspaper has accused Roger Liddle, one of Tony Blair's most senior advisors, of collaborating with a former Labour aide-turned-political lobbyist, who is said to have offered to sell companies ministerial access.

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The prime minister's official spokesman said the Downing Street Chief of Staff, Jonathan Powell, would be writing to Observer Editor Will Hutton for a third time to call on him to release any tapes it had recorded of Roger Liddle.

"If we have not heard by 11am tomorrow (Tuesday) that there is a tape and The Observer will release it with a full transcript to help clear this situation, we will assume there is no such tape," the spokesman said.

The Downing Street ultimatum follows a demand by Commons Leader Ann Taylor that tapes of alleged conversations between journalists and the accused lobbyists and aides are handed over to the police.

The Observer's claims have also been firmly denied by the government. The prime minister has stood by Mr Liddle, while Chancellor Gordon Brown has flatly rejected claims that details of his keynote and market-sensitive Mansion House speech had been leaked by another firm of lobbyists before he delivered it in June.

Suspended and sacked

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The denials came after the man at the centre of the allegations - an ex-adviser to senior minister Peter Mandelson - was suspended from his job as a lobbyist with GPC Market Access and sacked as a columnist with the Daily Express.

Derek Draper is accused of working with Downing Street aide Roger Liddle to offer clients privileged access to ministers - once they were paid.

The Observer newspaper also said Mr Draper - who is on holiday in Italy - had told an undercover reporter he had passed on details of the government's spending plans before they were officially announced.

Tories attack 'grossly improper' acts

Maude: "grossly improper"

The decision led to renewed demands for Mr Liddle also to be suspended from his job as European affairs adviser to Tony Blair and for a full Commons statement and investigation into the claims.

Mr Liddle has denied any wrongdoing and the newspaper insisted it did not accuse him of leaking any information.

The prime minister's official spokesman insisted Mr Liddle had been asked about the allegations, but had not been suspended and would not be.

He also said there would be no investigation into Mr Liddle's behaviour unless evidence of wrongdoing was produced, in which case Mr Blair would take it extremely seriously.

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But the Tories, who were engulfed in the "cash-for-questions" scandal before the last election, demanded a full investigation.

Shadow Chancellor Francis Maude said it was "grossly improper" for former aides to leak "stolen" information to their clients.

'Insider dealing'

He said claims that Chancellor Gordon Brown's Mansion House speech had been leaked by another lobbyist amounted to "insider dealing".

"What is improper is for people with intimate government connections to trade on that, to use it to get inside information, to give preferential advantage to their own clients, by sliding them under-the-counter information, advance copies of speeches, which are stolen," he said.

But Mr Brown hit back in a letter to Mr Maude insisting he had not produced "one detail of factual evidence" to support the claims the speech had been leaked.

Mr Brown said he had not finished writing the speech until very shortly before it was delivered and added: "I must therefore ask you now to confirm that you have no substantiated evidence for your claim that the Mansion House speech was leaked in advance."

Back the allegations

Leader of the House Anne Taylor underlined the challenge, saying no action could be taken until the newspaper had come up with evidence to back the allegations.

"Serious allegations have been made. We've been told by The Observer that they have got tape recordings of things that Roger Liddle has said. In that case they should put those tapes in the public domain."

Meanwhile, one of the other lobbying groups named in the newspaper wrote to the chairman of the Commons Trade and Industry Select Committee to apologise for leaking its report on energy policy 24 hours before it was released, claiming it was a "lapse of judgement".