LONDON, Jan 6 (Reuters) - British Prime Minister Boris Johnson apologised for not disclosing an exchange of messages with a Conservative Party donor to the head of an investigation into how his flat refurbishment was funded, letters published on Thursday showed. Johnson"s redecoration of his Downing Street residence has come under intense scrutiny, both for how much it cost and who paid for it. It is just one of several scandals to have dogged the prime minister at the end of last year. The redecoration by high-profile London interior designer Lulu Lytle has not been seen publicly, but has been described in media reports as involving expensive gold wallpaper and garish furniture. A donor initially paid tens of thousands of pounds of invoices, and Johnson later repaid the donor after he became aware of the arrangement through media reports. While Johnson has been cleared of breaking the rules, the episode has been used by critics to question his ethics, adding to a list of other incidents which have put Johnson"s judgment in the spotlight. Opposition politicians have accused him of running a corrupt government that hands out favours to cronies. He denies this. In a letter to Christopher Geidt, the independent adviser on ministers" interests, Johnson said he could not personally recall the exchange and "security issues" meant he had not been able to access an old device. "I am sorry that the Office of the 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 wrote in a Dec. 21 letter published on Thursday. The so-called "missing exchange" came to light in a separate investigation by the electoral watchdog. The watchdog fined Johnson"s Conservative Party over incorrectly declared donations which helped fund the flat refurbishment. read more Geidt said in a Dec. 17 letter that after reviewing the message exchange, he had not changed his initial conclusion, published in May, which cleared Johnson of a conflict of interest but criticised him for not scrutinising the funding. Geidt added more general criticism in the letters published on Thursday, singling out the processes that meant he had not seen the messages and saying that withholding information from him for any reason was unwise. "The episode shook my confidence precisely because potential and real failures of process occurred in more than one part of the apparatus of government," he said in a separate reply to Johnson dated Dec. 23.