A High Court judge has launched a new investigation into Boris Johnson’s decision to ban an anti-gay ad from London buses.

Mrs Justice Lang will investigate claims that Johnson was politically motivated in his decision to ban the ad in April 2012, due to his re-election campaign at the time.

The advert, paid for by the Christian charity Core Issues Trust, read: “Not Gay! Ex-gay, post-gay and proud. Get over it!”.

The High Court initially upheld the ban last year, but in January, the Court of Appeal ruled that the case must be re-opened, after internal emails from the mayor’s team came to light.

The High Court’s initial judgement was made on the basis that the decision came from within Transport for London, but an email from Johnson’s media chief at the time of the ban read: “Boris has just instructed TfL to pull the adverts and I’ve briefed the Guardian, who will break that news in next half hour.”

On Wednesday, Mrs Justice Lang said: “I do have concerns about the lack of full disclosure by Transport for London (TfL) and the lack of candour about the decision to bar the advertisement.

“I am still not satisfied the full story has been told.”

It is possible the mayor and his team will be called before the High Court to give evidence.

The investigation is unlikely to see the advert re-instated, as Lord Justice Briggs noted in January that even if the High Court eventually rules in favour of overturning the ban, Transport for London can “nonetheless lawfully decide again to refuse to accept the Trust’s advertisement”.