Boris Johnson has offered a “humble and sincere apology” to his standards adviser for not disclosing an exchange of messages with a Tory peer where he discussed the funding of the refurbishment of his Downing Street flat.

The Prime Minister told Lord Geidt, his adviser on ministerial standards, that the messages had been on an old phone which he had to stop using due to “security issues” and therefore he “did not have access to my previous device and did not recall the message exchange”.

But Lord Geidt said the failure to alert him to the WhatsApp messages exchanged with Tory donor Lord Brownlow had been “unwise”.

He said the incident had “shook his confidence” while Mr Johnson admitted: “A fuller explanation of the circumstances should have been provided at the time of your investigation. I am sorry we did not do so.”

It comes after Lord Geidt previously cleared the Prime Minister of breaching the code in relation to the funding of the flat refurbishment – a judgment he upheld on Thursday.

The Electoral Commission fined the Conservatives £17,800 after it found the party had not followed the law over donations by Lord Brownlow to help cover the renovations, with costs exceeding £112,500.

The watchdog said the Tories had failed to “accurately report a donation and keep a proper accounting record” of the money handed over by the peer in October 2020, but as part of the investigation it was revealed Mr Johnson had exchanged messages with Lord Brownlow in November 2020.

Mr Johnson had previously said he had no knowledge of the payments until immediately prior to media reports in February 2021.