Police are probing leaflets handed out on the Tube on election day by activists apparently endorsing a group which claims gay people can be 'cured'.

The pamphlets, which featured a picture of the Mayor of London, were distributed to commuters on the Metropolitan Line, and carried support for Christian charity the Core Issues Trust.

Boris Johnson banned adverts which the group booked to appear on London buses which read: "Not gay! Ex-gay, Post-gay and Proud. Get over it!"

The adverts were a reference to gay rights charity Stonewall's slogan “Some people are gay, get over it!”

A High Court judge cleared Mr Johnson of being politically motivated in banning them.

But the row was rekindled as voters went to the polls for the general election, in which the Mayor completed a sensational Commons return by being elected as MP for Uxbridge and South Ruislip.

The leaflets - which were headlined 'Is Boris lying?' - accused Mr Johnson of saddling the charity with a £100,000 legal bill.

They were reportedly placed on seats on the Metropolitan Line, which leads to Boris Johnson's Uxbridge constituency.

@PatrickStrud @pinknews Partner was given flyer by 'ex-gay' charity on the tube. Legal? Cleared by TFL? pic.twitter.com/6SBr93qZPM — Jon Dryden Taylor (@jondrytay) May 7, 2015

Such distribution on election day could be a breach of election laws, and may also breach Transport for London rules if permission to hand out material had not been gained.

The British Transport Police now say the leaflets are being investigated.

Duty Officer Anthony Capon told BuzzFeed: "We are aware of an issue relating to the distribution of politically motivated leaflets on rail services on the Metropolitan line during the course of Thursday, 7 May.

"We thank those who have brought the issue to our attention and will now seek to investigate the matter in the most appropriate manner."

The Core Issues Trust is a Northern Ireland-based Christian group which claims to offer support for "men and women with homosexual issues who voluntarily seek change in sexual preference".

The Evening Standard has approached both the Core Issues Trust and Boris Johnson's office for a comment.