Boris Johnson has been forced to issue a “humble and sincere apology” for mislaying texts about his flat makeover.

In letters published on Thursday, Johnson earned a strong rebuke from his own ethics adviser, Christopher Geidt, for his failure to disclose messages between himself and Tory donor David Brownlow, who contributed £52,000 towards the refurbishment of his flat above 11 Downing Street.

However, he said he stood by his original findings in May that Johnson had not broken the ministerial code.

“The new disclosure did not in fact result in change to my original assessment of your interests insofar as they related to the ministerial code,” he said.

In his report into the flat refurbishment published in May, Geidt reported that Johnson said he did not know that Brownlow had donated the £52,000 from his own pocket until shortly before media reports circulated in February 2021.

However, a separate probe into the flat makeover by the Electoral Commission discovered that Johnson had in fact messaged Brownlow over WhatsApp about the revamp in November 2020.

In a sharply-worded letter, Geidt told the prime minister that his failure to disclose the texts exposed a “signal deficiency in the standards upon which the independent adviser and, by extension, the prime minister have an absolute right to rely in establishing the truth in such matters”.

He added: “Indeed, the episode shook my confidence precisely because potential and real failures of process occurred in more than one part of the apparatus of government.

“I am very grateful to have your apology for these shortcomings and to know of your determination to prevent such a situation from happening again.”

What the texts to Brownlow said

Johnson described parts of his Downing Street flat as “a bit of a tip” as he solicited authorisation for renovations to his official residence

The WhatsApp messages show the PM trying to hurry Brownlow to give the go-ahead as he was “keen” for luxury interior designer Lulu Lytle to progress the work.

On November 29 2020, the prime minister wrote: “Hi David I am afraid parts of our flat are still a bit of a tip and am keen to allow Lulu Lytle to get on with it. Can I possibly ask her to get in touch with you for approvals?”

Later that day, Brownlow replied: “Sorry for the delay I was out for a walk and didn’t have my ‘work’ phone with me. Of course, get Lulu to call me and we’ll get it sorted ASAP!”

He subsequently added: “I should have said, as the Trust isn’t set up yet (will be in January) approval is a doddle as it’s only me and I know where the £ will come from, so as soon as Lulu calls we can crack on.”

In his letter to Geidt, dated December 21, Johnson offered a “sincere and humble apology” for his failure to submit the texts to his investigation, which he said was due to a change in his phone number that meant that he “did not have access to my previous device and did not recall the message exchange”.

And he said it was “unacceptable” that the Cabinet Office did not inform Geidt of the existence of the messages.

He said Brownlow had offered to share the messages with the Cabinet Office but that the department had felt this was inappropriate to receive them while the commission carried out its investigation.

“It is unacceptable that the Cabinet Office did not at the very least inform you of the position they had taken,” the prime minister said.

Following the letter exchange Labour deputy leader Angela Rayner accused Johnson of breaking the ministerial code and of offering “pathetic excuses”.

“Boris Johnson has little regard for the rules or the truth,” she said.

“The ministerial code requires ministers to act with transparency and honesty.

“It is simply impossible to read these exchanges and conclude that the prime minister has not breached these aspects of the code.

“Once again, by attempting to hide the truth, Boris Johnson undermines his own office.

“The prime mInister’s pathetic excuses will fool no-one and this is just the latest in a long line of sorry episodes.”

The Geidt report centres on a row over how the prime minister’s flat above 11 Downing Street was funded.

Reports suggest that the final bill may have topped £200,000, but with the PM only receiving an annual grant of £30,000 for such renovations, the remaining sum needed to be made up somewhere.

Enter Tory peer and donor David Brownlow, who was found to have made a £52,000 donation go help transform the flat — from a so-called “John Lewis nightmare” into a luxury Lulu Lytle abode.

However, Johnson originally failed to declare the donation in line with parliamentary rules, leading critics to demand an investigation over fears of a conflict of interest.

Two probes were then kicked off: one by the Electoral Commission and one by Geidt himself.

In his initial investigation into the flat makeover in May 2021, Geidt found that Johnson had “acted unwisely” by commissioning it without full knowledge of how it would be paid for, but cleared the PM of a conflict of interest.

He said Johnson did not breach the ministerial code by failing to declare the donation, and noted that this was eventually done by the prime minister — even if late.

Geidt also said that Johnson appeared not to be aware Lord Brownlow had contributed from his own pocket, saying he was told by the PM that he “knew nothing about such payments until immediately prior to media reports in February 2021”.

Geidt was forced to take another look at the flat saga after central findings he came to appeared to be contradicted by the Electoral Commission’s parallel probe.

The commission fined the Conservative party nearly £18,000 for failing to properly report a donation of £67,801.72 from Brownlow’s Huntswood Associates Limited in October 2020, including £52,801.72 connected to the costs of refurbishment to 11 Downing Street.

It also revealed that Johnson sent Brownlow a WhatsApp message in November 2020 requesting additional money for the refurb, in an apparent contradiction of what he told Geidt last May.

Geidt was reportedly left furious that the Whatsapp exchange had not been submitted to his investigation and was potentially on the verge of resigning.

According to the Sun, Johnson has blamed a change in his mobile phone number — after it was made freely available online — for the non-disclosure of the messages with Brownlow.