John Prescott broke rules for ministers in not immediately declaring his stay at a US tycoon's ranch, say MPs.
Mr Prescott still faces questions about a possible conflict of interest
The Commons standards committee said no action should be taken against the deputy prime minister as he had now registered the visit.
But the rules must change so future abuses can be properly investigated.
The Tories said MPs had given Mr Prescott more than just "a slap on the wrist" and there should now be a full independent inquiry.
Tony Blair has previously rejected calls for a such a probe saying he has seen no evidence that ministerial rules had been broken.
But shadow culture secretary Hugo Swire, who first raised questions about Mr Prescott's conduct, said: "If Mr Blair shies away from this then frankly the ministerial code will not be worth the paper it's written on."
Mr Prescott said he "accepted" the findings of the MPs' report and Downing Street says the problems have now been resolved.
The row centres on Mr Prescott's stay with US tycoon Philip Anschutz, who is bidding for a licence to build the UK's first and only super-casino at the Millennium Dome.
Mr Prescott has strongly denied there was a conflict of interest, saying he was not involved in decisions about the Dome.
The report said Mr Prescott initially appeared to have been in breach of the Ministerial Code.
It said he had taken the view that "accepting Mr Anschutz's hospitality would not place him under any obligation".
"However, what Mr Prescott failed to do at that time was also to address, as the Ministerial Code requires, whether the proposed hospitality was from a source which might reasonably be thought likely to influence ministerial action."
Mr Prescott had "come to accept" that the nature of his relationship with Mr Anschutz "meant he was accepting hospitality from a source that might reasonably have been thought likely to influence ministerial action".
That is the key test for deciding whether hospitality given to ministers has to go into the MPs' register of interests.
Parliament's watchdog, Sir Philip Mawer, said Mr Prescott had been "right" to register the trip - even if it had been 11 months late.
But he said: "the key question, which is one for the ministerial code, is whether he should have accepted the hospitality in the first place.
"That is a matter on which neither the standards and privileges committee or I can express a point of view."
Sir Philip - and the standards committee he reports to - are only allowed to investigate the rules for MPs, not those for ministers.
Sir Philip Mawer says public confidence is at risk
The MPs' report calls this state of affairs "confusing" for the public.
Committee chairman Sir George Young said the lack of an independent inquiry into breaches of the ministerial code meant "awkward questions are left hanging in the air".
Mr Blair would be better to ask for an investigation rather than "just wishing the whole thing would go away", said Sir George.
The MPs' report also confirms that Mr Prescott was given a cowboy outfit during his stay at the ranch.
The deputy prime minister says he was given a Stetson hat, a pair of calf length boots, a belt and buckle, a pair of spurs and a pair of jeans worth about £600 in total.
Sir Philip says he is not satisfied by the way Mr Prescott expected these gifts to be declared.
In a statement, Mr Prescott said he fully accepted the watchdog's report.
"I have registered the stay at the ranch," he said.
"The gifts, which were recorded at the time, will be notified in the annual return to Parliament next week, as is the usual practice."
He added: "I and my department also accept Sir Philip's concern about the procedures operated within my department for reporting gifts.
"I have asked my department to undertake an urgent review of its procedures, and indeed we have already begun to implement new procedures."
Mr Prescott's friend, Labour peer Lord Snape, said there was no evidence for a ministerial code inquiry.
The MPs had issued the "mildest possible rebuke" against Mr Prescott, who had taken advice from his top civil servant about following the code, added Lord Snape.
But Liberal Democrat culture spokesman Don Foster said a "cloud of suspicion" would hang over Mr Prescott unless the prime minister ordered an inquiry.