Lecturers are threatening to boycott the Oxford Union after its invitation to the controversial "racist" historian David Irving.

The Association of University Teachers is calling on the academic community to withdraw its support for the debating society.

Mr Irving - who has claimed the Holocaust never happened - is due to debate the issue of free speech at the Oxford Union on May 10. The event has been condemned by anti-racist groups and the National Union of Students.

David Triesman, general secretary of the AUT, has written an open letter to Oxford Union president Amy Harland.

He said: "By denying the terrible atrocities committed during the Holocaust, David Irving has denied the experiences and freedoms of others. He has denied the elimination of a whole generation of Jews, homosexuals, gipsies and other dissidents.

"If the Oxford Union feels compelled to allow him to appear, then I must inform you that this would leave me with no other option than to call for an academic boycott of the union both in this country and from among the academic community and other trade unions throughout the world."

Oxford Union spokesman Richard Coates said: "This debate is not about the Holocaust. This is a debate about free speech.

"I am surprised that the association can take this extreme opinion without contacting us and having it clarified. If they are so concerned why did they not contact us and receive a balanced view?"