Harry Enfield has been 'quietly no platformed' by the Oxford Union debating society because he defended using 'black face' to portray Nelson Mandela, a study has found.
Other speaks who have been 'no platformed' by the debating society include former Scottish First Minister Alex Salmond for 'past comments or views held'.
Enfield said he had 'no regrets' about blacking up to play Nelson Mandela in a play in 2007.
Sir Tony Blair found himself rejected as a speaker by its politics society following fears he could be 'picketed' by people who considered him a 'war criminal'.
These findings are from a study conducted by the Higher Education Policy Institution think tank, where students told researchers that they felt it would breach their 'duty of care' to invite the former prime minister to speak as it could upset students who might have had relatives who fought in the Iraq war.
The study has labelled it 'quiet no-platforming' - a phenomenon where speakers are rejected over fears they might cause offence.
Author of the report, Josh Freeman, said that students “are shying away from difficult topics and controversial speakers because they fear a backlash.”
“It is deeply concerning that one of the most important British Prime Ministers since the Second World War, who was democratically elected three times, should be quietly no-platformed,” he added.
“However controversial Blair may or may not be, how are students supposed to learn or decide what they believe if they are not even exposed to the full range of mainstream ideas?”
A spokesman for the Cardiff Politics Society said it was "not aware" of the decision not to invite Sir Tony but researchers said not all current committee members had been involved so they might not know about it.
A Cardiff University spokesman said: ‘The decision not to invite a speaker would be for our student Politics Society to take. The university was not involved.
Mr Enfield and Sir Tony Blair have been contacted for comment.