Boris Johnson has been criticised by the MPs standards watchdog for failing to promptly explain how a trip to Mustique was funded – however he has been cleared of breaching the rules.
An inquiry was launched into the prime minister's 2019 Caribbean holiday after confusion over who paid for the trip.
The Standards Committee has now concluded that Mr Johnson's account was "accurate and complete".
But, it added it was "regrettable" a full explanation had not come earlier.
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Mr Johnson declared that his £15,000 holiday accommodation on the Caribbean island had been covered by Carphone Warehouse co-founder David Ross.
Mr Ross initially said he had not paid "any monies" for the trip. He later clarified that he had "facilitated" accommodation for the prime minister.
Following an investigation, the standards committee concluded that Mr Ross had donated the accommodation, but added that the arrangements had been "ad hoc and informal and do not appear to have been fully explained to Mr Johnson at the outset".
"This matter could have been concluded many months ago if more strenuous efforts had been made to dispel the uncertainty," the committee said.
"It is regrettable that a full account and explanation of the funding arrangements for Mr Johnson's holiday accommodation has only come to light as a result of our own inquiries rather than at an earlier stage.
"If greater clarity had been made available to the commissioner at the first instance, this matter could have been cleared up many months ago."
"Given that Mr Johnson was twice reprimanded by our predecessor committee in the last Parliament in the space of four months for 'an over-casual attitude towards obeying the rules of the House', we would have expected him to have gone the extra mile to ensure there was no uncertainty about the arrangements."
A breach of the rules?
In May 2020, the parliamentary commissioner for standards, Kathryn Stone, undertook the investigation on behalf of the committee.
She found that Mr Johnson had "sought and was offered" use of a villa owned by Mr Ross, a Conservative party supporter and friend of Mr Johnson.
However, Mr Ross' villa was unavailable for the dates of Mr Johnson's holiday so instead another villa was found for the prime minister.
The commissioner said Mr Johnson was right to name Mr Ross has a donor, but said if another person had provided funds, Mr Johnson should also have given the details of whoever funded his holiday accommodation in the first instance.
While she did not find that Mr Johnson's account was inaccurate, she found the prime minister had breached the code because he did not "make sufficient inquiries" to establish who was providing his accommodation.
On receiving the commissioner's report, the committee concluded it did not have sufficient evidence to judge if the code had been broken.
However after making further inquiries – including contacting the owner of the villa where Mr Johnson stayed – it said it reached a different conclusion from the commissioner and concluded that Mr Johnson had not breached the rules.