A senior Tory MP has resigned his life membership of the Oxford Union in protest at the debating society's invitation to holocaust denier David Irving and BNP leader Nick Griffin.

The Oxford Union is under mounting pressure to withdraw its invitation to the two controversial figures for tonight's freedom of speech event.

About 1,000 anti-fascism and minority rights campaigners are planning to gather outside the debating chamber to jeer the arrival of Mr Irving, who was jailed for Holocaust denial, and Mr Griffin, who has been convicted of race offences.

Shadow defence minister, Dr Julian Lewis, said the students should be "ashamed" of themselves.

In a letter to the Union's officers and standing committee, Dr Lewis, MP for New Forest East, said he was resigning his life membership "with great sadness".

He wrote: "Nothing which happens in tonight's debate can possibly offset the boost you are giving to a couple of scoundrels who can put up with anything except being ignored.

"They have been exposed and discredited time and again by people vastly more qualified than you in arenas hugely more suited to the task than an undergraduate talk-shop, however venerable.

"The only good to have come from this self-indulgent behaviour is the fact that Muslim and Jewish students are working together to condemn the appalling message you have sent to their communities."

The invitation drew fire from Trevor Phillips, the Equalities and Human Rights Commission chairman.

He said: "This is not a question of freedom of speech, this is a juvenile provocation."

A Thames Valley police spokesman said this evening's demonstration would be policed "like any other protest" and Oxford's police chief, Chief Supt Brendan O'Dowda has warned troublemakers to stay away.

Oxford Union president Luke Tryl said there would be other speakers to attack the views of Mr Griffin and Mr Irving.