The openly-gay divorced father-of-two was consecrated in 2003 amid protests from traditionalists.

Prayed long and hard

He was due to speak on 11 March, proposing the motion "This House believes a gay lifestyle should be no bar to becoming a bishop".

But he said in a statement he had thought and prayed long and hard about the invitation.

He said: "It has become clear to me that for me to participate would not be in the best interests of the Anglican Communion at this delicate moment in its history. "

The Reverend Richard Kirker, general secretary of the Lesbian and Gay Christian Movement, was due to speak with Bishop Robinson

He said he was deeply saddened by the decision, "all the more so as it seems likely to all he was put under pressure to withdraw".

"It will seem strange to all fair minded people that while others discuss and debate him in a very personal and often offensive way, he finds himself forced into silence," he said.

"The Church has once again shot itself in the foot."

'Gagging order'

The Oxford Union, which attracts high-profile speakers ranging from farmer Tony Martin to Hollywood star Clint Eastwood, said the cancellation was a blow to free speech.

Oxford Union President Edward Tomlinson said: "It is a shame the archbishop's newly appointed commission should act as a gagging order, rather than as a catalyst for discussion.

"I understand that Bishop Robinson is a figure of totemic importance in this debate, and his love for the Anglican Communion has meant for the time being he believes silence to be the best course of action.

"As the president of the most famous debating society in the world, and as a committed Anglican, I look forward to the day when free discussion of this matter can and does take place."

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