UN Secretary General Kofi Annan was "entirely wrong" to oppose the expected US assault on the Iraqi city of Falluja, David Blunkett has said.
The US believes there are thousands of insurgents in Falluja
In letters to the leaders of the US, UK and Iraq, Mr Annan said it risked alienating Iraqis whose support was needed for next year's elections.
The home secretary told BBC Radio 4's Any Questions that terrorists had to be "rooted out" before the elections.
The US believes Falluja is housing several thousand insurgents.
American forces are pounding suspected rebel areas on the city's outskirts and a major attack is thought to be imminent.
Mr Blunkett said: "Obviously we have to listen to the secretary general of the UN, but I think on this occasion he is entirely wrong.
"If the elections are to take place, if they are to be adjudged to be democratic and if there is to be security and stability and a democratic Iraq in the future then the terrorists have to be rooted out."
Apr 2003: US paratroopers shoot dead 13 demonstrators
Nov 2003-Jan 2004: attacks on three US helicopters kill 25
Feb 2004: 25 killed in attacks on Iraqi police
31 Mar 2004: four US contractors killed
Apr 2004: US seals off city
May 2004: Siege lifted
June 2004: Zarqawi loyalists targeted in US raids - continuing to date
Oct 2004: Iraqi PM threatens military action if Zarqawi is not handed over
Shadow chief secretary to the Treasury, George Osborne, said: "If you leave the rebel stronghold of Falluja in place then I think it is very difficult to hold any elections in Iraq at the beginning of next year."
And Liberal Democrat Ed Davey said that "while the Americans have got to go about this in a way to minimise civilian losses, I don't think there is an alternative".
Mr Annan had written that the use of force could "reinforce perceptions... of a continued military occupation".
But a US spokesman at the UN dismissed Mr Annan's intervention.
"This issue is for the government of Iraq and those who are willing to help the people of Iraq," Richard Grenell said.
"Spectator nations and international organisations should do more in Iraq and not in New York," he told the AFP agency.
It is less than two months since the secretary general caused a storm by saying that the invasion of Iraq was illegal.