John Prescott is under further pressure after it emerged he was given a "cowboy outfit" by tycoon Philip Anschutz during a visit to his US ranch.
John Prescott's contact with a US tycoon is being investigated
According to the Mail on Sunday, Mr Prescott received a Stetson hat, cowboy boots and an initialled belt.
A spokesman for the deputy prime minister said all gifts would be registered by the end of the month.
A standards watchdog wants an inquiry to see if Mr Prescott broke the ministerial code of conduct.
The Conservatives have also written to the cabinet secretary, Gus O'Donnell, raising the same question.
Mr Prescott came under the spotlight after it emerged he and some of his civil servants stayed at the billionaire's Colorado ranch in July last year as part of a nine-day trip to America. A decision on the location of the first super-casino, allowed under the Gambling Act passed last year, is expected at the end of the year.
The Dome, which Mr Anschutz owns, is among the sites short-listed for the casino.
According to the paper, Mr Prescott was given the "cowboy outfit", reportedly worth £11,000, by Mr Anschutz during last year's trip.
Under the ministerial code, "no minister should accept gifts, hospitality or services from anyone which would, or might appear to, place him or her under an obligation".
Sir Alistair Graham, chairman of the Committee on Standards in Public Life, says Tony Blair should order an investigation into Mr Prescott to "clear the air".
He says Mr Blair should ask Sir John Bourn, head of the National Audit Office, appointed by the prime minister to advise on ministerial interests, to look at the matter.
"If there are ongoing media stories which are dominating the front pages of newspapers which have standards issues ... then, if the prime minister is seen not to give them sufficient personal attention or use the procedures that he has set in place, then he is in danger of coming under serious criticism for these matters," he told BBC Wales.
Shadow culture secretary Hugo Swire says "profound" questions remain over Mr Prescott's visit to Mr Anschutz's ranch.
He has written twice to Sir Gus asking him to clarify whether Mr Prescott has breached the ministerial code on accepting gifts or hospitality during his contacts with the tycoon.
Meanwhile, Downing Street has dismissed reports that Environment Secretary David Milliband is set to replace the deputy prime minister.
But Liberal Democrat leader Sir Menzies Campbell said the "drip, drip, drip" of information about Mr Prescott would influence his decision about whether to stay.
"I would not be at all surprised if we find him stepping down from the deputy prime ministership but remaining as the deputy leader of the Labour Party in order not to provoke the kind of leadership frenzy and the speculation over Mr Blair's future," he said.