“No-platform” policies under which students have tried to stop well-known figures such as the writer Germaine Greer and the activist Peter Tatchell speaking on university campuses are potentially illegal, according to advice commissioned by the National Union of Students (NUS).

The 37-page legal opinion, by Christopher McCall QC, says such policies are legal only if applied to members of proscribed groups such as terrorists. In other cases, they breach section 43 of the Education Act 1986, which requires universities and by extension student unions to ensure freedom of speech.

Colin Riordan, vice-chancellor at Cardiff University, who had to step in to insist Greer be allowed to speak after students tried to muzzle her , said: “The fundamental principle is that universities need to be places