British troops have assisted an operation to surround the Iraqi city of Basra and close the border with Iran.
Operation Troy involves 1,200 British troops
A British military spokeswoman said UK forces had secured checkpoints as part of the Iraqi-led operation to support the country's new security plan.
This follows reports that radical Iraqi Shia cleric Moqtada Sadr and many of his supporters have fled to Iran.
The spokeswoman said the aim was "to halt smuggling and contain criminal and rogue militia operations".
She added two nearby border crossings had been closed for 72 hours, after instructions from the Iraqi government.
Mr Sadr and key figures from his Mehdi Army, which has been identified by the US military as one of the key threats to law and order in Baghdad, have reportedly already crossed the frontier.
Codenamed Operation Troy, the mission began at 0600 local time (0300 GMT) on Thursday and involved British troops, Iraqi police and the Department of Border Enforcement, BBC News has been told.
It will continue for three days.
Troops from 19 Light Brigade assisted with the "lockdown", assisting Iraqi police in the task of checking every vehicle on the eight routes into and out of the city.
British military spokesman Maj David Gell said the move was linked with a security crackdown that began on Wednesday.
He told BBC News: "This is part of the same operation.
"Specifically in our area, in Basra, it will involve vehicle checkpoints, security operations that involve overt patrolling, enhanced point-of-entry security and patrols supported by coalition air assets if required."
Capt Ollie Pile of the 19th Light Brigade told BBC News forces involved in the move, codenamed Operation Troy, had been led by 2,000 Iraqi troops and included 1,200 British soldiers.
He added eight main routes into city were now subject to vehicle checkpoints, with all traffic stopped and searched.
Boat patrols are also in operation along waterways , with the Royal Marines searching traffic along the Shatt Al Arab waterway.
Security patrols are also in force within the city.
Capt Pile said the purpose of the move was to prevent the movement of weapons and munitions.
He added: "We see this as another step along the way towards the Iraqi authorities taking responsibility for the security of Basra."
Of the four provinces round Basra, two have already been handed over to Iraqi security.
The Iraqi and US security crackdown has already seen the closure of crossing points to Iran and Syria. Three to Syria and four to Iran will reopen after 72 hours, but others will close indefinitely.
Weapons permits have been suspended in Baghdad to all but Iraqi and US forces and registered security companies, the capital's nightly curfew extended by an hour and stop-and-search powers increased.