Boris Johnson has been criticised by his standards adviser over an official inquiry into the refurbishment of his Downing Street flat.
Mr Johnson said he was sorry that messages he had exchanged with a Tory peer who initially paid for the revamp had not been disclosed to Lord Geidt.
The messages were revealed during a separate inquiry by the elections watchdog on how the works were funded.
Lord Geidt said it demonstrated “insufficient” respect for his role. He added they would not have changed the verdict of his own investigation last year that Mr Johnson had not breached the ministerial rulebook. But, in an exchange of letters with Mr Johnson, he said they would have led him to question some of the conclusions of the report, published in May.
Replying to Lord Geidt, Mr Johnson offered a “humble and sincere apology”, and said the messages were sent from a previous mobile phone, which he did not have access to, and he “did not recall the message exchange”.
Mr Johnson’s text exchange with Lord Brownlow, published on Thursday, shows on 29 November the PM asked the Tory peer whether he could ask an interior designer contracted for the work to make contact with him to approve fresh redecorations.
Lord Brownlow had been asked by Mr Johnson to oversee a charitable trust to fund the work - an idea that was later abandoned.
Mr Johnson told Lord Brownlow that parts of his official residence, where refurbishment work began in April 2020, were “still a bit of a tip” and he was keen to allow the designer, Lulu Lytle, to continue with the renovations.
A couple of hours later, Lord Brownlow replied, and then in a subsequent message, he added that the trust had not yet been set up but “approval is a doddle as it’s only me and I know where the £ will come from”.
In his original report, Lord Geidt cleared the PM of a conflict of interest, after it emerged that Lord Brownlow had donated £52,000 to help cover costs.
He said the prime minister appeared not to be aware that Lord Brownlow had initially paid for the redecorations from his own pocket.
But he revisited the original probe after it was revealed the PM had sent WhatsApp messages to the Tory peer in November.
The messages were revealed as part of an investigation by the Electoral Commission, which fined the Conservatives £17,800 last month for failing to accurately declare Lord Brownlow’s donations.
After the exchange came to light, Downing Street said Mr Johnson had thought Lord Brownlow was overseeing the money, but did not realise the peer was providing the funds himself.
In his letters to the PM, Lord Geidt said he had been told Mr Johnson lost the ability to search for the messages in April 2020, after security changes following news his personal mobile phone number was available online.
However, he said it was “plainly unsatisfactory” he had not been made aware of their existence during his original investigation.
He also expressed “grave concern” that officials in Mr Johnson’s team had not alerted him to them when the phone was later accessed in June 2020, or before the Electoral Commission disclosed their existence in December.