Speaking at the Cop26 climate change summit in Glasgow, the Prime Minister said that if MPs were to carry on taking second jobs it was essential they abided by the regulations.
His warning came as the former attorney general Sir Geoffrey Cox said he did not believe he had breached rules which ban MPs from using their parliamentary offices for outside business.
Mr Johnson, who has seen the Tories’ poll ratings fall after the row over standards erupted last week, said he was not going to comment on individual cases, insisting “the UK is not remotely a corrupt country” .
Labour has referred Sir Geoffrey to the Standards Commissioner Kathryn Stone after video emerged of him apparently taking part in legal proceedings in the British Virgin Islands remotely from his office in Westminster.
MPs who break Commons rules must be “punished”, Boris Johnson has said, as he attempted to draw a line under the politically toxic row over standards at Westminster.
“The most important thing is that those who break the rules must be investigated and should be punished,” he told a news conference.