Parliament's standards watchdog reportedly believes the Prime Minister's holiday to Mustique was worth more than double the £15,000 he declared in the Commons register.
Kathryn Stone, the commissioner for standards, has also said the bill had not been met by Tory donor David Ross as Boris Johnson has insisted, according to reports in the Daily Mail.
The revelation will heap further pressure on Mr Johnson as he faces various investigations into whether he properly declared any donations to cover the lavish refurbishments of his official flat.
Downing Street insisted the PM "transparently declared the benefit in kind" of the luxury Caribbean holiday, and noted that Mr Ross confirmed the declaration is "correct".
The Parliamentary Commissioner for Standards confirmed this week she is still investigating whether Mr Johnson properly declared the holiday on the private island 16 months ago.
In the Register of Members’ Interests, the PM declared the trip with fiancee Carrie Symonds as a "benefit in kind" from the Carphone Warehouse founder who has a villa on the island.
But the Mail said Ms Stone believes the break was worth more than twice the declared £15,000.
Mr Johnson was said to have refused to accept the ruling and is trying to have it overturned to avoid the risk of being suspended as an MP.
A Downing Street spokesman said: "The PM transparently declared the benefit in kind in the Commons Register of Interests. The Cabinet Office was aware of the declaration and was content it was appropriate.
"A spokesman for Mr Ross confirmed the PM’s declaration is correct and the accommodation was facilitated as a donation in kind.”
This week, a spokesman for Mr Ross said in a statement: "Mr Ross facilitated accommodation for Mr Johnson on Mustique valued at £15,000.
"Therefore this is a benefit in kind from Mr Ross to Mr Johnson, and Mr Johnson’s declaration to the House of Commons is correct."
Labour called on Mr Johnson to "come clean" with the public over the funding of the holiday.
Deputy leader Angela Rayner said: "The public have a right to know who paid for his luxury holiday, how much they paid for it and what they might expect in return for their generosity.
"If these reports are true, the Prime Minister will once again have shown his complete contempt for the basic standards of integrity in office.
"The Prime Minister has already broken parliamentary rules regarding his financial interests. It is clear that he thinks that the rules that apply to the rest of us simply do not apply to him."
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