The decision to pull troops out of Iraq will be independent of the United States and based only on the "British national interest," the Foreign Secretary said today.

David Miliband said that the Government would decide whether to leave the southern city of Basra based only on the "situation on the ground".

The Foreign Secretary, however, reiterated the position set out by Gordon Brown last night, who ruled out an immediate timetable for the withdrawal of troops in a letter to Sir Menzies Campbell, the Liberal Democrat leader.

It came as The Times reported this morning that Sir Richard Dannatt, the Head of the Army, had ordered his senior staff to make preparations for a "generation of conflict" in a speech that the Ministry of Defence tried to keep secret.

The Foreign Secretary said that the situation in Basra was a "very difficult, very tough, very challenging situation for our troops," but added that it was "different" to the situation in Baghdad.

Asked to confirm that whatever was decided in relation to Baghdad by President Bush would not affect British decisions, he told BBC Radio 4's Today programme: "Absolutely. Our decisions about Basra are about the situation on the ground in Basra, not the situation on the ground in Baghdad.

"Clear objectives, clear criteria for when they are met, and clear command structures based on the assessment of the situation on the ground by the real experts who are sitting on the ground."

He added: "We will always take British decisions in the British national interest."

As Mr Miliband spoke, however, the prospect of UK troops being able to ensure stability in Iraq became all the more remote when the Basra security chief predicted "some disorder" when any withdrawal from the city took place, and demanded a larger number of Iraqi Army divisions be sent there.

Hakim al-Miyahi, the head of the security committee on Basra City Council, told the AP news agency that Iraqi security forces in the area are too few to compensate for the loss of British forces, and that it would take "at least two army divisions" to fill the gap.

The British are expected to pull their last troops out of Basra city centre soon, leaving all UK forces stationed at the Basra International Airport.

Meanwhile, the Prime Minister last night rejected a fresh call by the Liberal Democrats to set a timetable for troop withdrawals from Iraq, amid a host of recent casualties.

In an open letter to Sir Menzies, he argued it was wrong to say the continued presence of British troops would achieve little, or to say that they were severely restricted in what they could achieve.

"UK forces in Basra continue to have the capability to strike against the militias and provide overall security," he said.

Mr Campbell had called for a timetable to pull out, saying casualty levels were now unacceptable, but Brown said this would "undermine our international obligations, as well as hindering... our Armed Forces and increasing the risks they face".

At least 41 British soldiers have been killed in southern Iraq this year - the highest number of British casualties since the first year of the US-led war in 2003, when Britain had a total of 18,000 troops in the country.

Currently, the UK has around 5,000 soldiers in Iraq, stationed mostly in and around Basra. Some 2,200 troops have pulled out in the past year, and British generals are believed to be gearing up for more pullouts in the coming months.

This morning, The Times reported that Sir Richard Dannatt, the Head of the Army, has ordered his senior staff to make preparations for "a generation of conflict".

The General gave warning of the dangers posed by a "strident Islamist shadow" and suggested that the British Army was "on the edge of a new and deadly Great Game in Afghanistan".

"The challenge of this generation is as great as any that have gone before us," he said.

His thoughts about the way forward for the Army were revealed in a speech given to a conference in London in June. The speech remained secret because the MoD did not allow the media to attend.

However, under a Freedom of Information request, the contents of the address to senior British and overseas military have now been released.

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