It has submitted a request for all the paperwork under the Freedom of Information Act.
Andrea Williams, director of the Christian Legal Centre, said: “Boris is increasingly under pressure for his actions and integrity. This is another example of where he has been expedient as opposed to doing truth and justice.”
Core Issues Trust, the Christian group which was behind the advert intended to run on 24 buses last year, has been granted permission to appeal to the Court of Appeal.
The Christian group argued that stifling debate by banning their advert amounted to discrimination.
They pointed in particular to one poster which some Christians found offensive. Funded by Richard Dawkins, the academic, and the British Humanist Association in 2009, it said: “There’s probably no God. Now stop worrying. And enjoy your life.”
A TfL spokeswoman said: “The advertisement clearly breached our advertising policy as it contained a controversial message and was likely to cause widespread offence to the public.
“We are taking steps to address the judge’s comments regarding our internal processes.”