Transport for London's (TfL) advertising policy has been denounced as "untenable" after an advert for a controversial appeared on .

A billboard promoting Scientology that encourages travellers to fund out more about the group was spotted over the weekend.

The religion, which is based on the work of L. Ron Hubbard, has been denounced as a "cult", has been the subject of controversy and legal action in multiple countries. Scientology has rejected these claims, describing itself as a "religion in the fullest sense of the word" on its official website.

@carbdiem posted a photo of the ad on . They claimed: "New development in questionable TfL advertising spaces: a literal cult."

Ben Harris reposted the picture and wrote: "Seriously - what the hell is wrong with TfL? Ads for Scientology are infinitely more damaging to the public than ads featuring burgers."

TfL does not allow foods that are high in fat, sugar or salt to be advertised on its network. In March, comedian Ed Gamble was forced to swap a hot dog for a cucumber in his Tube ads.

Now, Emma Best AM, the Deputy Leader of City Hall's Conservatives, had told : "TfL's advertising policy has become untenable.

"In recent months, extremely controversial adverts have been placed across the network promoting Scientology, the assisted dying campaign and a hate preacher while theatre and comedic shows featuring wedding cakes and hot dogs have been banned.

"It is ridiculous TfL has defended placing adverts on the network which the Advertising Standards Agency have ruled should not be allowed to run.

"It is the Mayor that chose to place himself as the moral arbiter of what is right and wrong in terms of TfL advertising and so it is right that he now accounts for the absurd situation we find ourselves in."

A TfL spokesperson said: "The advertisement was reviewed against our advertising policy and was found to be compliant."

A spokesperson for Sadiq Khan that the Mayor of London "has no involvement in approving or deciding which ads run on the TfL network, and TfL's policy reflects legal requirements."