Boris Johnson ‘broke electoral law’ if he failed to declare donation for flat makeover within one month

Boris Johnson has broken electoral law if he received a gift or loan for his expensive flat makeover and failed to declare it within one month, he has been warned.

The strict rules were set out amid claims that the Conservative Party secretly approved paying the £58,000 bill as long as nine months ago – a payment then covered by a Tory donor, according to a leaked email.

“The whole purpose of this is we need to know who’s paying for politicians,” said David Howarth, a former Electoral Commissioner, “so all these transactions need to be looked at”.

The professor of law at the University of Cambridge said the law requires politicians to declare quickly any gifts or loans that “benefits them in connection with any political activities”.

If they failed to do so, they were “in breach of a rule that says they have to report certain donations or loans within a month”, he told BBC Radio 4.

Mr Johnson, and other ministers, have repeatedly failed to deny that the money originally came from the party or a donor – while insisting the prime minister has now paid it.

But the full money trail is likely to be revealed when an updated register of ministerial interests is finally released – many months late – possibly as early as this week.

“The person who has to report is the donee, the money to whom the money is given or loaned,” Professor Howarth said – not the donor – adding: “The standard is a month.”

Gus O’Donnell, the former Cabinet Secretary, also warned the prime minister appeared to be “very late” in complying with the law in the saga over the redecoration of his Downing Street flat.

“Prime ministers have to set an example and, therefore, they should abide by the rules. I think that’s really important,” he said.

Mr Johnson is believed to have scrambled for help to pay for the refurbishment – apparently demanded by his fiancée Carrie Symonds, to rip out Theresa May’s ‘John Lewis furniture nightmare’ – after the costs ran “out of control”.

The decor was said to have been inspired by Lulu Lytle, a celebrated eco-interior designer, who makes furniture “based on traditional crafts”, it was reported.

On Monday, the new Cabinet Secretary Simon Case confirmed Mr Johnson explored setting up a charitable trust to pay for the flat’s refit, above an allowance of £30,000.

He said it was now clear that would be illegal – but refused to say whether political donations had been accepted to help fund the project.

The prime minister again ducked the question yesterday, saying: “If there’s anything to be said about that, any declaration to be made, that will of course be made in due course.”

The updated register of ministerial interests will only be released when a new adviser on ministerial standards is in place – a post due to be filled last week, but also delayed.