The deal would permit up to 100 British troops to remain beyond June 30, the withdrawal date set in a previously approved British-Iraqi security pact.
A source close to deputy speaker Khalid al-Attiya said that the session was suspended shortly after it began when lawmakers loyal to the cleric, who galvanized Shi'ite opposition to the U.S.-led foreign presence in Iraq, walked out before a planned vote on the deal.
With no quorum, the session could not formally continue, but parliament is likely to take it up again at a later date.
Aqeel Abdul-Hussein, head of the Sadr bloc in parliament, told reporters afterwards that Sadr supporters opposed any agreements backing the presence of foreign troops. The deal "is an extension of occupation forces which no noble person can accept ... We call upon our people to support us in this challenge," he said.
Both Britain and the United States reached deals with the Iraqi government, which were approved by parliament, to allow their troops to remain in Iraq after a United Nations mandate expired at the end of 2008.
The U.S. deal requires the withdrawal of the approximately 130,000 U.S. troops from Iraq by the end of 2011.
Britain, which sent 46,000 troops to the Gulf for the 2003 invasion, had withdrawn its 4,000 solders to the airport in the southern city of Basra, Iraq's southern oil hub, by 2007. It began to pull out for good earlier this year.
British officials say remaining forces will focus on protecting oil platforms off Iraq's southern coast, where most of Iraq's oil exports are shipped, and on training Iraqis.
Iraq is struggling to revive its ailing oil industry and is courting some of the world's biggest energy firms to help it boost production now around 2.3-2.4 million barrels per day.
Violence has dropped sharply in Iraq, which has the world's third largest oil reserves, but insurgents continue major attacks. Oil has been a frequent target for attacks since 2003.