Iraqi authorities have insisted that a southern city has not been taken over by Shiite militants, even as UK forces were put on standby to move in.
British soldiers were "ready to act" to support domestic forces in Al Amarah following a series of violent clashes, the Ministry of Defence said.
Members of anti-American cleric Muqtada al-Sadr's Mahdi Army reportedly stormed three main police stations and flattened the buildings with explosives.
The attack comes just two months after the city was considered calm enough to be handed over to domestic control amid moves to transfer the whole of Maysan province to the control of Iraq forces.
A senior Iraqi politician sent to the area by Iraqi Prime Minister Nouri Maliki has denied reports that the militia was in complete control.
The situation was said to be "calm but tense." At least 15 people are reported to have been killed and 59 wounded.
An MoD spokeswoman said: "A reserve battalion of Iraqi police has been deployed. Multi-national forces are ready to act in support of security forces if required."
Maysan and Basra are the only provinces that remain under British control.
Liberal Democrat leader Sir Menzies Campbell said: "This worrying development only serves to underline the failing nature of the strategy in Iraq.
"Unless a more effective strategy can be put in place the continued presence of British forces cannot be justified."
