Labour has confirmed that its proposed cap on outside earnings for MPs could cost Boris Johnson tens of thousands of pounds if he returns to the House of Commons.
Ed Miliband is considering imposing a strict cap of around £15,000 on all MPs' outside earnings. He also wants to change the law to ban all MPs holding paid directorships or consultancies if Labour wins power.
The idea is to make sure all MPs are properly committed to their job. It follows the "cash for access" row that has engulfed senior MPs Jack Straw and Sir Malcolm Rifkind.
The earnings cap would hit Boris Johnson in the pocket if he became an MP because he wants to remain London mayor until his term ends in May 2016. Under rules already in force, he will lose two-thirds of his £143,911 mayoral salary if he enters the Commons – taking it to around £47,970.
But Labour's proposed cap means Johnson could have to forfeit £32,970 of that.
However, he would still get his £67,060 MP's salary on top, plus an estimated £250,000 a year for writing a weekly column in the Daily Telegraph and royalties from his books. Labour is not planning to stop MPs earning money from media appearances, books, and articles.
Labour insisted its plans for a cap on outside earnings were at a "very early stage" and no exact limit had yet been decided.
But a senior party source said the mayor's pay packet – even when depleted by two-thirds – would be "almost certain to breach the cap". They said the aim was to deter MPs from ever holding the position of mayor at the same time.
A source close to Johnson said that if he won the election, he intended to take both his MP's salary and his reduced mayoral salary. They claimed that changing the rules on Johnson's pay would require primary legislation plus an amendment to the Greater London Authority Act.
Labour MPs and candidates have already been warned that party rules have changed so they can no longer hold paid directorships or consultancies. The party will hold a House of Commons debate and vote on the issue on Wednesday.
"We need to act to improve the reputation of our political system in the eyes of the British people," a Labour spokesperson said. "MPs are dedicated to the service of their constituents and the overwhelming majority follow the rules. But the British people need to know that when they vote they are electing someone who will represent them directly, and not be swayed by what they may owe to the interests of others."
MPs declared earnings of more than £7.4 million from outside work last year, according to the Telegraph. The jobs ranged from executive directorships and advisory roles to TV appearances.