Theresa May has repeatedly declined to answer questions about whether Defence Secretary Gavin Williamson told the truth about an extra-marital liaison.
Mr Williamson dramatically confessed last week that he "almost threw away" his marriage by kissing a female colleague while working at a fireplace manufacturing firm before he entered Parliament.
The 41-year-old South Staffordshire MP said the flirtation progressed no further than kissing and that his wife forgave him after he owned up and apologised.
Asked during a visit to China whether she was confident that Mr Williamson had told the truth about what happened, and about his subsequent departure from the company Elgin and Hall, Mrs May said only: "Gavin has told people about something that happened before he became a Member of Parliament, let alone became a minister."
Journalists travelling with the PM pressed her on the issue, pointing out that she had sacked Damian Green as deputy prime minister because he was shown not to have told the truth about events which happened before he joined the Government.
Mrs May replied: "What has come out is something that happened even before Gavin was a Member of Parliament, let alone a minister.
"In relation to Damien Green, the report showed the ministerial code had been broken, it was on that basis that I took the decision."