Boris Johnson faces the threat of tough sanctions from the House of Commons standards watchdog if it is confirmed he took a loan to pay for refurbishment of his Downing Street flat.

The prime minister was facing growing pressure over the £88,000 refit, with Labour demanding an investigation into whether Mr Johnson’s former press secretary misled the media by denying that Conservative Party funds were used and whether there was a “cover-up” by senior members of the government.

And, in a letter obtained by The Independent, Liberal Democrat leader Sir Ed Davey has called on the head of the civil service Simon Case urgently to reveal whether any donor received anything in return for helping the PM with the lavish renovation of the No 11 apartment.

A No 10 spokesperson attempted to kill off the growing controversy by repeating Friday’s statement that Mr Johnson had personally paid for any spending above the £30,000 permitted annually from the public purse, adding: “Conservative Party funds are not being used for this.”

But critics said that key questions remain to be answered, as the statement did not exclude the possibility that party money was initially used but had now been returned by the PM, or resolve the issue of whether Mr Johnson had been forced to take out a loan in order to repay the cash.

If the money was originally received as a loan, the PM could fall foul of rules requiring him to update the register of MPs’ interests within 28 days.

And he is already under a warning that he faces a tough punishment for further breaches of the rules – even including suspension from the Commons – after twice previously being admonished for filing interests late.

In a report in 2019, shortly before Mr Johnson became PM, parliamentary standards commissioner Kathryn Stone revealed that he had missed the deadline for registering a financial interest in a property in Somerset, just months after apologising for a delay in declaring outside earnings and assuring her that his records were now up-to-date.

Ms Stone said the late declaration showed a “pattern of behaviour” which could be seen as a “lack of respect” for Commons rules. And the cross-party Committee on Standards warned that any repeat of his “over-casual attitude” would be treated as “a matter which may call for more serious sanction”.

As Mr Johnson has already apologised to the commissioner and the Commons for previous lapses – and been ordered to meet with the registrar of interests to have his obligations as an MP explained – the next most serious sanction could involve a recommendation for suspension, though this would almost certainly be rejected in a Commons vote.

One Conservative former minister told The Independent that Tory MPs were less concerned about voters’ views of the propriety of the flat refit than about the impact of the PM appeared to regard himself as above the rules.

“Outside the metropolitan bubble, there’s not an enormous sucking of teeth about his accommodation,” said the MP. “If anything, voters are puzzled as to why the PM has to pay for the wallpaper in his office, and a lot of them think ‘good old Boris’.

“What people on the Tory benches are worried about is not the flat, it’s this idea that he has an entitlement to be above the rules because he’s Boris. If people are playing fast and loose with the conventions, that is very dangerous.”

Meanwhile, Scottish Conservative leader Douglas Ross three times declined to say that he regarded Mr Johnson as a man of “honour and integrity” in an ITV interview.

A survey by pollsters Ipsos Mori found that a massive 59 per cent of voters do not regard Mr Johnson as “trustworthy”, compared to little more than a third (35 per cent) who do not. By comparison 42 per cent regard Labour leader Keir Starmer as trustworthy and 41 per cent do not. David Cameron, whose lobbying on behalf of finance firm Greensill kicked off the latest “sleaze” furore, was seen as untrustworthy by 74 per cent.

However, Mr Johnson’s overall satisfaction ratings were unchanged and Ipsos Mori head of political research Gideon Skinner said: “As previous leaders have shown, you don’t have to be ahead on trust to win elections if the public thinks you have other strengths.”

In his letter to Mr Case, Sir Ed Davey said there were “legitimate concerns” about the ultimate source of the money used to refurbish the flat.

Leaked emails suggest that Tory donor Lord Brownlow paid the Conservative Party £58,000 last year, to cover payments to party had made to a planned trust intended to look after the funding of future renovations, which he was expected to chair.

But the trust was not established, and Downing Street today said that a review being conducted by cabinet secretary Mr Case and expected to take a few weeks would look into whether it was an option.

Details of any loans or gifts to the PM would normally be published in the twice-yearly register of ministerial interests. But no register has been published since July last year, and Downing Street says this cannot happen until the appointment of a new independent adviser to replace Sir Alex Allan, who quit in November after Mr Johnson overruled his finding that Priti Patel bullied Home Office staff.

Sir Ed said that any delay in making details of the funding public was “unacceptable”, telling the cabinet secretary: “You must act decisively to clear up any questions about: the source of this money; whether anyone who contributed funds for this purpose received anything from the prime minister in return; and what action wil be taken if any wrongdoing is found to have been committed.”

Ms Stratton told reporters on 8 March that “Conservative Party funds are not being used to pay for any refurbishment of the Downing Street estate.” And on 26 March, Tory chair Amanda Milling said that party funds “are not being used” to pay for the flat, but declined to say whether they had previously been used. In a letter to Mr Case, Labour’s deputy leader Angela Rayner demanded an investigation into “whether the former press secretary knowingly misled journalists and the public, or was misled herself by senior members of the government who seem intent on a cover-up”.

A No 10 spokesperson said: “Any costs of wider refurbishment this year beyond those provided for by the annual allowance have been met by the prime minister personally. Conservative Party funds are not being used for this.”