Greenpeace confronted HMS Vanguard off Florida

But the Trident-armed Vanguard submarines build up far more tritium than the Trafalgar-class vessels that have been refitted at Plymouth.

The Environment Agency agreed to allow the dockyard operating company, DML, to increase emissions, but the final approval rests with the UK government.

The agency said the amount of radiation involved was lower than existed naturally in the area, and was well within national limits.

As part of the consent, DML was ordered to reduce all radioactive emissions by cutting back on the amount of cobalt discharged.

But the pressure group Campaign Against Nuclear Storage And Radiation, representing people living on both sides of the Tamar, said the river was being poisoned.

Legal doubt

Green Party MEPs have raised objections with the EC.

The EC has written to the UK government to ensure the consent complied with European legislation.

Trident submarines were built at Devonport

It has received no response but is continuing to investigate.

The Environment Agency said normal rules did not apply because the refits were a military operation.

A large police operation is planned for the arrival of the port's first Trident-armed Vanguard class submarine.

HMS Vanguard was expected from Sunday onwards.

A protest by about 500 anti-nuclear and environmental campaigners will begin at Devonport Park at noon on Sunday.

Shore guard

Police leave has been cancelled, with between 200 and 300 officers assigned to line the shore to prevent anyone entering the water to reach the 150-feet submarine.

Police at Faslane, where the submarines are based, have given advice about protests, and the Special Branch has been consulted about the risk of terrorism.

Campaigners oppose tritium emissions at Devonport

HMS Vanguard will not be carrying its 16 Trident missiles.

Neighbouring police forces have been alerted so more resources can be brought in if needed.

Chief Superintendent Steve Pearce said: "This is a major operation for Devon and Cornwall Constabulary.

"Our role is to enable life to go on and ensure the safety of the general public and the protesters we anticipate coming here."

Devonport beat Scotland's Rosyth dockyard to the �5bn refit contract in 1993, but is more than a year behind schedule and �180m over budget in preparing for the work.