UK troops were deployed a week after the offensive began
The British in Basra made a secret pact with the Mehdi Army which kept the military out of March's Iraqi-led offensive against the Shia militia for a week, according to the Times newspaper.
The BBC's Crispin Thorold, in Baghdad, assesses whether such an "accommodation" could have been possible.
In March this year the Iraqi security forces launched a major offensive against the Mehdi Army, a Shia militia, in Iraq's second city Basra. From the beginning the British described that operation as "Iraqi planned, led and executed".
But once again questions are being asked about why the British were so slow to put their troops on the ground in the city.
From the earliest hours of the Iraqi military operations in Basra it was clear that things were not going according to plan.
The resistance by Shia militiamen was much stronger than had been anticipated.
Yet British troops were only deployed from Basra's airport into the city after nearly a week of fighting.
Could that decision have been dictated by a secret deal between the British and the Mehdi Army, as suggested by the Times?
The newspaper has claimed that UK troops initially stayed out of the battle because of a pre-arranged "accommodation" with the Mehdi Army - denied flatly by the Ministry of Defence.
However, closer examination of the British relationship with the militias in Basra shows that such a deal could have been possible.
Military intelligence sources have told the BBC that the British have been talking to Shia militias including the Mehdi Army for several years.
At times the frequency of the talks have declined, like during the Shia uprising in 2004.
More recently there appear to have been specific deals between the two sides.
Ali al-Salman, a senior commander of the Mehdi Army in Basra told the BBC that he attended three meetings with a "British army officer and a British 'civilian' between 8 February and 10 February 2007".
According to the militia commander, a colonel from the Iraqi Army, and Ahmad Al-Fartusi, another member of the Mehdi Army who had been detained by the British, were also present.
Our attempts to find Ahmad al-Fartusi, who is now in hiding, have been unsuccessful. The Iraqi Army refused to comment.
Ali al-Salman claims that under the terms of the deal 60 Shia prisoners were released, the British agreed to stop patrolling in Basra and the Mehdi Army agreed not to attack the British headquarters in the city.
The Mehdi Army commander argues that the British "didn't keep their part of the bargain as [in April 2008] they participated in military operations in Basra".
A British military spokesman in Basra refused to discuss the details of any negotiations with any party.
"Nothing of that nature affected movement into the city", said Major Tom Holloway.
If there were any sort of accommodation between the Mehdi Army and the British army the details are unlikely to have been widely disseminated.
"If the delay in fighting in March/April 2008 was due to a deal the only people who would have known would have been the defence minister, the defence chief, a very senior commander in the field and some planners," said a British military intelligence source.
"The command has to maintain morale. It can't be seen to do a deal with people who have killed soldier's mates. That would make the government as popular as a bag of puke," said the source.
Whatever the truth there are still serious questions about why the British took so long to support the Iraqi army when they were on the back-foot.
"British forces in Basra were postured for training," said Major Tom Holloway, the British military spokesman in Basra.
"They were on the streets by 1 April when they had turned the training mission into a support mission."
Observers reject that outright.
There are more than 4,000 British forces in the Basra airport, including three or four infantry battle groups.
Even at a conservative estimate that would mean 1,800 British soldiers would be ready to fight.
Could the delay have been due to a political reluctance in Whitehall for British soldiers to be embroiled in an Iraqi fight just months after the British had handed over security control of the province?
Again the answer to that is unclear.
The 60,000-strong Mehdi Army was created in 2003
The American military though was not as slow to respond.
Their extensive involvement in the battle for Basra was revealed by the BBC in April 2008.
It is now clear that 1,000 American soldiers were on the streets of the city within 48 hours.
Compare that to the British response.
The US military is said to be "slightly at a loss with the British".
There is a perception among some Americans that the decisions that were taken were a direct result of an "accommodation" with the militias.
Either way the military consequences are now being seen in Basra.
"There are a lot of discrete American units operating in the city", said a military intelligence source.
"They are considering sending as many as 15,000 troops down there".