The UK has been retraining the Iraqi river patrol service

A Royal Navy training team has been playing a key role in retraining and re-equipping the service, which was disbanded after the war.

Original personnel have been taught how to handle high-speed inflatable boats.

With coalition forces, they are monitoring the waterway for smugglers, the UK Ministry of Defence said.

They will eventually take over the security completely.

The aim of the programme has been to safeguard maritime traffic on the strategic highway between Iraq and Iran.

It also aims to establish good conditions on the border for both Iran and Iraq so that vessels can travel freely without being threatened by robbers or thieves.

The waterway is considered a major smuggling route for contraband oil as well as a crossing point for foreign militants trying to infiltrate Iraq.

Source of war

The Royal Navy training team had been delivering a boat from Umm Qasr, where similar training programmes have been taking place, to Basra, when the incident happened.

A UK defence ministry spokesman said the two Boston Whalers and one British Army Combat Support Boat seized by Iran on Monday may have strayed across the maritime border by mistake.

The arrested crews had been delivering the Combat Support Boat to the new Iraqi Riverine Patrol Service, he said.

The boats were unarmed but the sailors were carrying their personal weapons at the time, the MoD confirmed.

The border between Iran and Iraq runs down the middle of the waterway.

The waterway is Iraq's main access to the Persian Gulf, and the source of the territorial row that led to an eight year war between Iran and Iraq in the 1980s.