Electoral Commission investigation launched into Boris Johnson's Downing Street flat refurbishment

ITV News Correspondent Romilly Weeks looks at just how costly the PM's curtains have become

Boris Johnson's refurbishment of his Downing Street flat is being investigated after it was found there are "reasonable grounds" to suspect an "offence or offences" may have occurred.

The prime minister's problems over his residence have dramatically worsened by the Electoral Commission's announcement that it will launch a "formal investigation" into the Number 11 flat's pricey renovation.

Questions have been mounting for Mr Johnson since former aide Dominic Cummings accused him of wanting donors to "secretly pay" for the renovations to his residence in a "possibly illegal" move.

Will this scandal stick? ITV News Politics Editor Robert Peston discusses

At PMQs, Mr Johnson insisted he paid for the refurbishment of the flat "personally", adding he would make any further declarations if necessary.

The prime minister refused to directly answer Sir Keir's questions on who "initially" made payments for the flat, after it was exclusively revealed by ITV News Political Editor Robert Peston that costs were covered in the form of a loan from the Tory Party.

Peston says Mr Johnson is now repaying his party for the renovations, which are reported to have cost tens of thousands.

Asked if he believed any offences had been committed, Mr Johnson said "no I don't".

He claimed the public would find it "absolutely bizarre" that Sir Keir was focusing on the refurbishment.

Mr Johnson could face a £20,000 fine if he's found to have broken the law - a cost that may not be welcome, given he reportedly told aides he could not afford the revamp as costs spiralled.

Prime ministers get a budget of up to £30,000 per year to renovate their Downing Street residency, but newspaper reports suggest up to £200,000 has been spent.

In the House of Commons, Sir Keir reminded Mr Johnson he is required to declare any benefits that relate to his political activities, including loans or credit arrangements, within 28 days.

Watch PMQs in full:

He added: "He will also know any donation must be recorded in the register of ministers' interests and, under the law, any donation of over £500 to a political party must be registered and declared. So, the rules are very clear.

"The Electoral Commission now thinks there are reasonable grounds to suspect an offence or offences may have occurred. That's incredibly serious."

It was the most tense session of PMQs in weeks, with Mr Johnson visibly rattled by the onslaught of accusations, which came from the SNP's Ian Blackford and backbench MPs after Sir Keir finished his questions.

Announcing its investigation, the Electoral Commission said it had "conducted an assessment" of information provided by the Conservative Party since contact began late last month.

"We are now satisfied that there are reasonable grounds to suspect that an offence or offences may have occurred," the statement said.

The Electoral Commission statement added: "The investigation will determine whether any transactions relating to the works at 11 Downing Street fall within the regime regulated by the Commission and whether such funding was reported as required.

"We will provide an update once the investigation is complete. We will not be commenting further until that point."

Before the probe was announced, a No.10 spokeswoman said costs for the flat beyond those provided for by the £30,000 annual allowance, had been “met by the prime minister personally”.

The spokeswoman added: “Conservative Party funds are not being used for this.”

Health Secretary Matt Hancock refused to answer questions about the investigation into the refurbishment of Downing Street during the government's coronavirus press briefing.

Speaking at the Downing Street briefing, Mr Hancock said: “I know that the Prime Minister answered lots of questions about this in the House of Commons earlier.

“And given that this is a coronavirus press conference, you won’t be surprised that I’m not going to add to the answers the Prime Minister has given.”

Labour has accused Mr Johnson of having "lied" over the funding, and accused senior members of the Government of a possible "cover-up".

Meanwhile, an appointment has finally been made to fill the vacant role of independent adviser on ministers' interests.

Former private secretary to the Queen Lord Geidt will take up the post, which has been vacant since Sir Alex Allan resigned in November after the PM overruled him in relation to a report on Priti Patel's conduct.

The appointment of the new adviser paves the way for the publication of the latest register of ministerial interests, which could contain details of any donations to fund the Downing Street flat.

Listen to our latest Calling Peston podcast: