Tony Blair is facing mounting pressure to order an investigation into whether John Prescott broke the rules on how ministers should behave.
The Liberal Democrats are writing to the prime minister to demand an inquiry into Mr Prescott's talks with the man who wants a super casino at the Dome.
And the Tories have accused the UK's top civil servant of undermining the rules by failing to investigate.
No 10 says Mr Prescott has explained his meetings with Philip Anschutz.
The deputy prime minister says he made sure he was separated from any decisions on planning applications or the casino bid for the Dome.
But Lib Dem culture spokesman Don Foster warned: "The prime minister must agree to undertake a full investigation of whether Mr Prescott broke the ministerial code, or suspicions of wrongdoing will linger."
Civil servants' mistake
Mr Blair is the ultimate arbiter of the ministerial code of conduct - although he sometimes asks Cabinet secretary Sir Gus O'Donnell to look into complaints.
But so far he has asked neither Sir Gus, nor his adviser on conflicts of interest, Sir John Bourn, to look into the controversy.
For the Conservatives, shadow culture secretary Hugo Swire wrote to Sir Gus asking him to investigate Mr Prescott's visit to the US tycoon's Colorado ranch.
In his reply, Sir Gus said Mr Prescott had now declared his stay at the ranch in the MPs' register of interests.
But the civil servants who had accompanied him on the visit had not logged the stay in the department's register of hospitality.
"I have ensured that this has now been done," said Sir Gus. "My view is that the officials should have been advised to do so at the time.
"However, I am satisfied that this has not affected their subsequent advice or actions."
Mr Swire said he was "astonished" Sir Gus had not cleared Mr Prescott but instead failed to investigate. He is writing back to ask for clarification.
"To treat such a serious allegation in this manner completely undermines the ministerial code," he said.
"Unless these concerns are taken seriously it will undermine any suggestion that the code in any way guarantees the standards of behaviour of government ministers."
A Cabinet Office spokesman said there was a very simple reason why Sir Gus had not mentioned the ministerial code.
"Paragraph 1.3 of the ministerial code sets out very clearly that it's not the role of the secretary of the Cabinet or other officials to enforce or to investigate ministers, although they might provide ministers with private advice on matters which the code covers," he said.
Parliament's standards watchdog Sir Philip Mawer has launched a full investigation into the ranch visit.
And Sir Alistair Graham, chairman of the Committee on Standards in Public Life, says Mr Blair risks serious criticism if he does not show he is giving personal attention to the allegations.