Boris Johnson has been quizzed over whether he acted with "honesty and integrity" in his relationship with US businesswoman Jennifer Arcuri.
The prime minister gave a single word reply: "Yes".
He was challenged about the issue by Huffpost Political Editor Paul Waugh at a Downing St Covid press conference.
The PM has faced renewed questions over his dealings with Ms Arcuri when he was London mayor. He has consistently denied any wrongdoing.
Ms Arcuri received £126,000 of public money in the form of grants for her technology business and event sponsorship. She was also given access to three foreign trade missions led by Mr Johnson.
Last year, the Independent Office for Police Conduct (IOPC) said it would not be launching a criminal inquiry into whether Mr Johnson abused his position as mayor to "benefit and reward" Ms Arcuri.
But it also said in a statement that Mr Johnson should have declared conflicts of interest.
The IOPC was asked to look into the matter because Mr Johnson, who was mayor between 2008 and 2016, was also in charge of policing in London.
At Tuesday's Downing Street press conference, Paul Waugh told Mr Johnson the current Greensill lobbying controversy had "sparked a lot of interest in whether the Nolan principles in public life have any teeth or relevance any more".
He asked the PM: "Do you agree with the Independent Office for Police Conduct in its review of your links with Jennifer Arcuri, which concluded 'it would have been wise for Mr Johnson to have declared this as a conflict of interest and a failure to do so could have constituted a breach of the Nolan principles?'
"Those principles involve acting with honesty and integrity.
"Do you believe you acted with honesty and integrity in your relationship with Miss Arcuri, who claims you conducted your affair in your marital home?"
Mr Johnson, who has divorced his second wife, Marina Wheeler, since the alleged events described by Ms Arcuri in recent interviews, gave a lengthy reply to Mr Waugh's first question, about a third runway at Heathrow Airport.
But he confined his remarks on the Arcuri question to a single word.
The Nolan Principles were established in 1994, under then prime minister John Major, in response to a series of scandals involving Conservative MPs.
The seven principles established by Lord Nolan, which are not legally enforceable, are: Selflessness, Integrity. Objectivity, Accountability, Openness, Honesty and Leadership.
Lord Evans, Lord Nolan's successor as the chairman of the Committee on Standards in Public Life, has told the BBC the standards system needs to be given more teeth, following revelations about Greensill Capital.
Mr Johnson has ordered an inquiry into the Greensill affair, which involves contacts between former PM David Cameron, former top officials, and a now failed finance firm.
The prime minister is still facing a Greater London Assembly investigation into whether he broke its code of conduct over his dealings with Ms Arcuri.
Downing Street has declined to say whether Mr Johnson will give evidence to the City Hall inquiry, but the PM's spokeswoman Allegra Stratton has previously insisted he acted in accordance with the Nolan principles.