The attack on a patrol boat in Basra that killed four British servicemen and injured three others will lead to an urgent review of UK forces' tactics in southern Iraq.
A lack of helicopters and the threat of roadside bombs has meant that the military have been depending more and more on the Shatt al-Arab waterway.
The Ministry of Defence has named the units of the troops who died. The men were from the Royal Signals, the Intelligence Corps, 539 Assault Squadron Royal Marines and from Commando Royal Marines attached to 539 squadron.
The bomb that ripped apart the boat on Sunday is thought to have been hidden on a pontoon at the confluence of the Tigris and Euphrates rivers.
The channel is quite narrow at some points leaving patrol boats vulnerable to gun and missile fire from the shore. Low pedestrian bridges criss-cross parts of the waterway, forcing boats to slow down.
Earlier this month most of the staff of the British consulate, on the banks of the Shatt al-Arab, were evacuated after regularly coming under mortar attack.
The waterway is a vital lifeline for commerce in Basra as well as a supply line for Iraqi rebels. It also forms part of the border with Iran. Six months ago an intelligence assessment claimed an increase in both arming and training of the militias by Iran. British forces will have to continue using the Shatt al-Arab.
Robert Emerson, a security analyst, said: "What happened was very worrying but it is, in many ways, quite surprising that such attacks have not taken place before. In reality the security options are quite limited and it is pretty well-nigh impossible to stop vessels from being targets."
No group has claimed responsibility for the attack.Reuse content