Britain’s most senior civil servant is personally overseeing the fallout from the refurbishment of Downing Street, Whitehall sources have told Sky News.

Simon Case, the cabinet secretary, faces criticism from opposition MPs and figures inside the government because of his personal involvement sorting out the financial arrangements surrounding the rebuilding of Downing Street.

Boris Johnson and Carrie Symonds, his fiancée, are said to have carried out a lavish overhaul of the flat above 11 Downing Street in order to replace Theresa May’s “John Lewis furniture nightmare”.

Number 10 has not said how the redesign, advised by eco interior designer Lulu Lytle was funded.

A leaked email this week suggested £58,000 in funding for a “Downing Street Trust” came from a Conservative donor, Lord Brownlow, although no donation has been declared to the Electoral Commission.

It is unclear how a trust could be used to fund last year’s refurbishment. The Tories deny breaking the law and Downing Street say no trust has yet been set up.

The Electoral Commission is already holding “discussions” with the Conservative Party over whether the work on Mr Johnson’s private flat was paid for by Tory donors and these payments should have been declared.

The Conservative Party said it complied with all the rules, but the Electoral Commission told Sky News in a statement the discussions with the party over “the works at 11 Downing Street” are still ongoing.

The government has already said that details about the Downing Street estate would be contained in the Cabinet Office annual report, due out just before the summer.

However, news of the personal involvement of Mr Case in the internal correspondence about the refurbishment has raised concerns inside government, with colleagues worrying it is a distraction given his huge in-tray because of the challenges of the COVID pandemic, Brexit and the government’s “levelling up” agenda.

There are also concerns that the issues around the flat could become more problematic if the discussions between the Conservative Party and Electoral Commission are not resolved satisfactorily.

“I’m astonished that Simon has got involved in this himself,” said one member of the government familiar with Mr Case’s involvement in the issue.

“It has so much potential to go wrong. What is he involving himself in something like this for? It’s so odd.”

Mr Case was appointed as the youngest-ever cabinet secretary last September, to replace Sir Mark Sedwill, after returning to Downing Street from the Royal Household.

Mr Case was close to Dominic Cummings, Mr Johnson’s former chief adviser.

Labour asked why Mr Case was involved in such an apparently trivial matter.

Shadow Cabinet Office minister Fleur Anderson said: “There are already serious questions hanging over the Downing Street refurbishment and this raises another that needs to be answered.

“It seems strange and misplaced for the prime minister to have tasked someone as senior as the cabinet secretary with a job like this in the midst of a pandemic, when there were surely far more pressing issues to deal with.”

According to the Daily Mail, Lord Brownlow of Shurlock Row, a Conservative donor, paid £58,000 to the Tories to replenish the party’s finances for an identical payment it had made towards the refurbishment.

He suggested that he would soon lead a new charity that would take charge of Downing Street’s upkeep.

In an email, leaked to the Daily Mail, from Lord Brownlow to Mike Chattey, the Conservative Party’s head of fundraising – which copied in Conservative co-chair Ben Elliot and Darren Mott, the party’s chief executive – he wrote: “Hi Mike, It was great to talk to you yesterday. Further to our conversation I am making a donation to the Party today as we discussed.

“It includes the £15,000 you and I have agreed plus £58,000 to cover the payments the Party has already made on behalf of the soon to be formed ‘Downing Street Trust’ of which I have been made chairman, as you know.”

No such trust has been formed and Number 10 has not promised there will be one, with officials saying the situation will be spelled out in the Cabinet Office annual report.

The £15,000 donation mentioned in the email was declared to the Electoral Commission, while the £58,000 did not appear.

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An Electoral Commission spokesman said: “Discussions with the Conservative Party continue as we work to establish whether any sums relating to the works at 11 Downing Street fall within the regime regulated by the Commission, and therefore need to be reported and subsequently published. The Party is working with us on this.”

A Cabinet Office spokesperson said: “The Downing Street complex is a working building, as well as containing two ministerial residences.

“As has been the case under successive administrations, refurbishments and maintenance are made periodically.

“Matters concerning works on the Downing Street estate, including the residences, will be covered in the Cabinet Office’s next annual report and accounts.”

A Conservative Party spokesman said: “All reportable donations to the Conservative Party are correctly declared to the Electoral Commission, published by them and comply fully with the law.

“Gifts and benefits received in a ministerial capacity are, and will continue to be, declared in government transparency returns”.

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