It has been a bad week in the normally quiet and mainly Shia south of Iraq, where British troops have been trying to maintain order since the end of the 2003 war.
By Jonathan Charles
BBC News, Baghdad
Now, the coalition and Iraqi authorities are wondering whether they are seeing the start of a fresh wave of violence in the run up to next month's referendum on the new Iraqi constitution.
A bomb outside a restaurant in Basra was the third attack in as many days.
Four security guards were killed in an earlier attack
Sixteen people were killed and 21 injured when a pick-up truck full of explosives was detonated near a restaurant popular with the Iraqi security forces. Among the dead were two children.
A short distance away, another car bomb was discovered by police officers who carried out a controlled explosion.
It was almost certainly meant to catch the emergency services as they tried to save people injured in the first blast.
Such tactics have been rare in Basra and are more commonly used by insurgents in the mainly Sunni north of Iraq, where American forces have had a hard time.
It has been almost a year-and-a-half since the south last suffered anything similar.
In April last year, 73 people died after suicide bombers carried out co-ordinated blasts at three police stations in Basra and the police academy in nearby Zubeir.
This latest explosion followed the deaths of four US security guards in Basra.
They were killed on Wednesday when their vehicle was hit by a bomb in Basra - hurling it off a bridge onto nearby land.
The contractors were providing protection for US diplomats in the city.
Two days earlier, two British soldiers were killed when their armoured Land Rover, part of a convoy, was targeted on a road close to their headquarters.
The attacks on both the British and the Americans were claimed by an al-Qaeda group operating in Iraq. It described the British soldiers as "crusaders".
Senior British sources have told me in recent weeks that they have not excluded the possibility of a rise in violence in the south during the campaigning for the constitutional referendum.
They will now be trying to ascertain whether these attacks are the start of that trouble.
They are also concerned that the insurgents may be changing their tactics.
Better explosives and detonators are thought to have been coming over the border from neighbouring Iran. They are not just enabling more numerous attacks, but also ensuring greater effectiveness.
British forces have been used to patrolling Basra without helmets, projecting an image of friendliness, believing that will win over the local population.
They will now be increasingly alert, hoping that the serious problems of northern Iraq are not beginning to seep south.