This makes the Basra a vital outlet to the Gulf for marine transportation of oil and fuel products – a lucrative prize for any political faction looking to consolidate its power in Baghdad.
The ensuing power struggle has led to an exodus of Sunni, Shia and Christian families northwards and often out of the country.
Earlier this week, Major-General Jalil Khalaf, commander of the Basra police department, admitted for the first time that the militias have proven too strong for – and often infiltrated - his forces.
Speaking to As-Sabah, the official Iraqi daily, he said: "Most of Basra's ports, especially Um Qasr, are under the control of militia gangs.
"The police force is incapable of executing its duties because its members report to Basra's militias and (political) parties which own those militias.
"Their loyalty is not to the Iraqi state but to their parties."
Khalaf was unavailable for comment.
Hoping to fill the power vacuum the British troop withdrawal has left in its wake and to diminish militia power, Nur al-Maliki, Iraq's prime minister, has promised that Iraqi forces are ready to take over security in southern Iraq.
But Nadim al-Jabiri, a senior politician from the powerful Shia Fadila Party, which maintains an iron grip over Basra, played down the deployment of any national military force in the south.
He told Al Jazeera: "If the Iraqi forces are really fit to preserve security in Iraq's far south, then it should be capable of doing so in the capital, Baghdad.
"But unfortunately, Iraqi forces are not being able to maintain order in the capital, how will they succeed in Basra?"
Abu Abd Allah, an Iraqi official who works closely with British forces in counterterrorism operations in the south, believes the Iraqi army is plagued by the same divided factional loyalties that have weakened the Basra police force.
"Iraq is still under foreign occupation and Iraqis continue to die in great numbers"
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He told Al Jazeera: "The government is getting ready to fill the gap by sending more troops to Basra. This will prove useless if most of the troops are loyal to certain parties and not to Iraq as a nation."
Basra's security began to deteriorate in 2006 when the Fadila party withdrew from the Unified Iraqi Alliance (UIA), the ruling Shia political bloc, in protest against the nomination of Nuri al-Maliki as a successor to Ibrahim al-Jaafari, the former prime minister.
Both al-Maliki and al-Jaafari are senior members of the Daawa party and Fadila had argued it was time a prime minister from another party be appointed to lead the country.
Throughout 2007, political infighting between Shia politicians in Baghdad plunged Basra further into chaos as clashes between the most powerful factions intensified.
Some Iraqi politicians are hoping to stave off the Shia infighting.
Fadila's al-Jabiri says his party is not looking for a fight.
"We are definitely not interested in any sort of confrontation, simply because we are the party who is successfully running Basra, and we would not spark things that would jeopardise our achievements in the city," he said.
"However, there are other parties that are interested in creating problems for al-Fadila in Basra."
Al-Jabiri declined to name those parties, but forces loyal to Muqtada al-Sadr and Abd al-Aziz al-Hakim maintain a strong presence in Basra.
Haroun Mohammad, an independent Iraqi politician and author, believes the British presence is currently the buffer holding back these factions from sparking an all-out Shia-on-Shia militia war in Basra.
He said: "Four representatives of Ali al-Sistani, Shia's highest religious authority, have been assassinated in southern Iraq in the past two months. They were killed in purely Shia dominated areas, where Sunni militias and the Iraqi resistance do not exist.
"All indications suggest that Shia militias killed those representatives." British mismanagement? But Abu Abd Allah faults the British military for failing to curb the activities of militias in Basra.
He said: "The British military's poor performance in curbing the growth of these gangs was the reason behind the militias' upper hand in the city. So, their withdrawal would not have a great impact on the city's already bad security."
Major Mike Shearer, spokesman for British troops in southern Iraq, denies that his troops have adopted a tolerant approach to militias in Basra.
|The pullout of British troops from the centre|
of Basra has left a power vacuum [EPA]
Shearer also expresses confidence in Iraqi troop redeployment in the south and the Iraqi army's ability to clamp down on terrorism and restore order in Basra city.
He told Al Jazeera: "Recently, we have noticed that Iraqi troops have been dealing with security breaches, without calling us for help, while in the past we used to be summoned every time a security challenge broke out."
But Iraqi politician Mohammad disagrees with the optimistic assessment of British military commanders.
"Shia on Shia fighting is a serious development," he told Al Jazeera.
"I think it is all about which militia is going to have the upper hand after the British withdrawal and eventually the rest of the southern Shia region."