Tony Benn, who retired from parliament in 2001, was one of Labour's longest-serving MPs. He has fought 17 parliamentary elections and served on his party's National Executive for more than 30 years.
His final speech as an MP earlier this year received a standing ovation. He has said he intends to devote more time to politics.
Last week Mr Benn said the US attacks in Afghanistan were "extremely worrying". He added: "It is an expected war. President Bush says there is a wide coalition but he has not sought the authority of the UN, so it is outside the charter and its legality is therefore questionable."
A cabinet minister in the Callaghan administration, Mr Benn is one of New Labour's fiercest critics.
He led an attack on the government's two-stage plan to abolish hereditary peers - preferring to abolish them outright. His father was a hereditary viscount, but Mr Benn fought to avoid inheriting his seat in the Lords, and by his challenge forced a change in the law.
He opposed the Falklands and Gulf wars and criticised Tony Blair's government over UK involvement in air strikes against Iraq and the ensuing sanctions.
Tony Benn was a strong contender for the deputy leadership in 1981 and has made two failed bids for the party leadership.
