Boris Johnson has offered a “humble and sincere apology” for failing to disclose WhatsApp messages with a Tory peer in which he discussed the funding of his Downing Street flat refurbishment – and blamed getting a new phone for not declaring the conversation.

In a series of letters between the prime minister and his independent adviser on ministers’ interests, Lord Geidt, it has been revealed that Mr Johnson cited “security issues” for him having not previously shared the messages.

In April last year, it was revealed that Mr Johnson’s personal mobile phone number had been freely available on the internet for the past 15 years, which forced the prime minister to change devices.

In the WhatsApp messages, from November 2020, the prime minister described the current state of his Downing Street flat as “a bit of a tip” and said he was keen to get interior designer Lulu Lytle to “get on” with the renovation.

Mr Johnson’s messages were sent to multimillionaire Conservative donor Lord Brownlow, who provided £112,500 for the revamp of the flat above Number 11 that the prime minister shares with his wife Carrie.

In response, Lord Brownlow – who was planning to set up a trust to fund the Downing Street refurbishment, although ended up initially paying the costs himself – told Mr Johnson he would “get it sorted ASAP”.

He added: “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.”

The WhatsApp messages were only revealed by an Electoral Commission investigation into the Downing Street flat row, and were not disclosed to Lord Geidt’s own probe into the matter.

Prior to the conclusion of the Electoral Commission investigation, Lord Geidt had himself cleared the prime minister of having breached ministerial rules with regards to the renovation of his Downing Street flat.

In his report, the crossbench peer – who was previously the Queen’s private secretary – said the prime minister told him he did not know how the refurbishment was being paid for until late February 2021.

In the letters published on Thursday, Lord Geidt expressed his anger that he was not made aware of the November 2020 messages when he investigated the matter; and which therefore left his report to be contradicted by the Electoral Commission probe.

He wrote to Mr Johnson that it was “of grave concern” that no attempt was made to retrieve the messages even after his report was published in June last year and when “the device was again accessed for another purpose”.

Lord Geidt told the prime minister that this provided “evidence of insufficient care for the role of your independent adviser”.

However, he added that the revelation of the messages does not alter his “fundamental assessment” that Mr Johnson did not breach ministerial rules in the funding of his flat refurbishment.

In his response to Lord Geidt, from 21 December, the prime minister wrote: “I am sorry that the office of independent adviser has been put in this position and can only repeat the humble and sincere apology I gave when we discussed this matter earlier today.”

He added: “You appreciate the security issues faced at the time meant that I did not have access to my previous device and did not recall the message exchange.”

However, some have questioned why the WhatsApp messages would have been lost if the prime minister had changed his personal phone number amid security concerns.

WhatsApp offers advice on how chat history can be preserved even if changing a phone number, or when using a new device.

Labour’s deputy leader Angela Rayner accused Mr Johnson of having “little regard for the rules or the truth”.

“The ministerial code requires ministers to act with transparency and honesty,” she said.

“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.

“This matters because it matters who has influence on our government in a democracy.

“The British public can’t WhatsApp a wealthy donor to open their wallets on request, and the least they deserve is transparency about who’s bankrolling their prime minister.”

Following the furore over the flat refurbishment, Lytle’s company Soane Britain refunded the Cabinet Office who, in turn, refunded the Conservative Party.

The Conservative Party had provided a “bridging loan” of £52,802 to the Cabinet Office for the works, before being reimbursed by Lord Brownlow.

The Tory peer also paid invoices worth £59,747.40 directly to Soane Britain.

The Conservative Party were last month fined £17,800 by the Electoral Commission for “serious failings” in neglecting to accurately report Lord Brownlow’s donation of £52,802.

Mr Johnson has since personally paid for the costs of the renovation, which is speculated to have included £840-a-roll gold wallpaper.