John Reid said British troops would not leave the job "half done" in Iraq

Mr Reid said UK troops would not stay in Iraq "any longer than is necessary" and the Iraqi government would judge how long that should be.

He was speaking after talks with Iraqi Prime Minister Ibrahim Jaafari about the security situation in the country.

Former foreign secretary Sir Malcolm Rifkind has meanwhile called Iraq a bigger "disaster" than Vietnam.

Sir Malcolm said Tony Blair should resign as prime minister over the issue of Iraq as it was "widely recognised" he went to war "on a false prospectus".

We stand by Iraq when times are tough and we will be a committed friend, not a fair-weather friend John Reid Defence Secretary Leaders firm after Basra unrest

The talks between Mr Reid and the Iraqi prime minister follow the events of Monday when the British army said it had to launch a rescue mission after two SAS men were arrested in Basra and handed to Shia militants by police.

The Iraqi interior ministry has disputed Army accounts that police handed the men over to Shia militants.

Following the London talks, Dr Reid said the "pace of transition" depended on how long the Iraqi government judged British forces were needed.

"We will not cut and run and we will not leave the job half done," he said.

"We stand by Iraq when times are tough and we will be a committed friend, not a fair-weather friend."

He also denied the government had been forced to delay plans to reduce the numbers of British forces in Iraq.

He said the plans remained "exactly the same" and troops would stay in Iraq until the local government developed sufficient forces to "counter terrorism and preserve the security of democracy".

I find it staggering that the prime minister remains in office Sir Malcolm Rifkind

Mr Reid also denied that Monday's incident in Basra meant British forces were no longer welcome in southern Iraq.

Tory leadership contender Sir Malcolm Rifkind called Iraq the UK's "most serious foreign policy disaster" in 60 years.

Sir Malcolm, who had opposed the Iraq invasion, told BBC2's Daily Politics show the Suez crisis was "modest in comparison" as it was over in a few days.

"Even Vietnam is not as bad as where we are now because at least in Vietnam the Americans had been invited in by a government trying to protect themselves from communism," he said.

'Insurgents'

The Kensington and Chelsea MP said the circumstances surrounding Iraq were worse.

"Here we actually went in, created a war that would otherwise not have occurred and now we see a complete political vacuum... I find it staggering that the prime minister remains in office."

Mr Rifkind also said Britain and the US should not "cut and run" but should recognise the scale of the error.

However, he said Britain should withdraw troops if it became clear Iraqi police in Basra and the south "had become not an ally but an enemy".

But the former foreign secretary said "we are not in that situation yet".

Iraq's national security advisor, Muwafaq al-Rubaie has conceded that security forces and police in "many parts of Iraq" had been penetrated by insurgents.

He told the BBC's Newsnight programme Iraq now had in place "a very scrupulous, very meticulous vetting procedure" to combat this.