MPs are planning to impeach Tony Blair for "high crimes and misdemeanors" in taking Britain to war against Iraq, reviving an ancient practice last used against Lord Palmerston more than 150 years ago.

Eleven MPs led by Adam Price, Plaid Cymru MP for Carmarthen East and Dinefwr, are to table a motion when parliament returns that will force the prime minister to appear before the Commons to defend his record in the run-up to the war.

Nine of the MPs are Welsh and Scottish Nationalists, including the party leaders, Elfyn Llwyd, and Alex Salmond, and two are Conservative frontbenchers, Boris Johnson, MP for Henley and editor of the Spectator, and Nigel Evans, MP for Ribble Valley.

A number of Labour backbenchers are considering whether to back the motion, though it could mean expulsion from the party.

The MPs' decision follows the commissioning of a 100-page report which lays out the case for impeaching Mr Blair and the precedents for action, including arguments laid down in Erskine May, the parliamentary bible, on impeachments dating back to medieval times.

The authors are Glen Rangwala, a lecturer in politics at Newnham College, Cambridge, and Dan Plesch, honorary fellow of Birkbeck College, London.

Under the ancient right, which has never been repealed, it takes only one MP to move a motion and the Speaker has to grant a debate on the impeachment. This means, at the least, Mr Blair will have to face a fresh debate on his personal handling of the war and there will have to be a vote in parliament on whether to institute impeachment proceedings.

In effect, impeachments were discontinued after Lord Palmerston, accused of concluding a secret treaty with Russia, survived an impeachment debate in 1848. The proceedings were replaced with a convention on ministerial responsibility, with ministers being forced to resign if they misled parliament. The last two cases involved the Home Office minister Beverley Hughes, over immigration clearances in Romania and Bulgaria, and Peter Mandelson over the Hinduja passports affair.

Mr Price said he believed the case was compelling. "To dust off Victorian constitutional histories and examine precedents from the time of Charles I and Chaucer may seem bizarre. But the conduct of the prime minister has left people and parliament with no alternative if we are to preserve the very basis of democracy."

© Guardian Newspapers Limited 2004

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