Gordon Brown had said he hoped to cut troops in Basra to 2,500 by spring
The number of British troops in Iraq will not be reduced as planned, due to violence in Basra, Defence Secretary Des Browne has told MPs.
Since October the government has cut troop numbers from 5,000 to 4,000. But plans for a further reduction to 2,500 have been halted, he confirmed.
During the weekend, forces became directly involved in fighting between the Iraqi army and Shia militiamen.
The Lib Dems have asked whether the role of UK troops in Iraq has changed.
'Prudent to pause'
Speaking in the House of Commons on Tuesday, Mr Browne said the government was still committed to reducing troop numbers, but recent events had prompted it to reconsider its plans.
"Before the events of the last week, the emerging military advice, based on our assessment of current conditions then, was that further reductions might not be possible at the rate envisaged in the October announcement - although it remains our clear direction of travel and our plan.
"In the light of the last week's events, however, it is prudent that we pause any further reductions while the current situation is unfolding.
"It is absolutely right that military commanders review plans when conditions on the ground change."
Future requirements would be assessed with coalition partners and Iraqis. Mr Browne said he expected to update MPs later in April.
'Show of force'
Iraqi government forces have been trying to wrest control of Basra and other Shia areas from the Mehdi Army - a Shia militia loyal to the radical cleric Moqtada Sadr.
UK troops have provided surveillance, flown fast jet missions over Basra as "shows of force" and used helicopters to help re-supply the Iraqi security forces, Mr Browne said.
He outlined recent involvement by the UK forces, saying tanks, armoured vehicles and artillery had been used to provide "in extremis" support to Iraqi units on the ground, while one of the Iraqi headquarters was resupplied by another UK battlegroup.
Des Browne (l) told MPs he would update them later in April
Logistic support was also provided in the supply of food, water and ammunition and medical care was given to wounded Iraqi personnel.
At the weekend, a British army spokesman said UK artillery had fired upon a mortar crew in the al-Khalaf area of northern Basra, which had attacked Iraqi soldiers.
It was the first time British troops had directly joined the fighting since the Iraqi army operation began on Tuesday.
Basra was taken by British forces in 2003. They withdrew from the city to the airport last autumn, and handed over security to Iraqi forces in December.
Responding to the announcement, shadow defence spokesman Liam Fox questioned the way British forces were being used in southern Iraq.
"It's surely not acceptable for us simply to end up mopping up, if we don't have a say in what operations are being carried out and how they're being carried out.
"It appears from what the Secretary of State has just told us that our commanders had only 48 hours notice (of the Iraqi offensive) and they yet had to deploy one battle group with tanks, armoured vehicles and artillery - is this an acceptable model for the future?"
The Liberal Democrat defence spokesman, Nick Harvey, asked whether the role of the troops had changed.
"The statement today again refers to the concept of 'overwatch', which I think people will previously have understood to have involved training, surveillance, logistic support, and being available on standby.
"But today he's told us about fast jet missions, and the deployment of tanks, armoured vehicles, and artillery. Is this really still 'overwatch' in the sense that will generally have been understood?"