The BBC's Tom Heap: "The ship is virtually unsinkable"

Environment groups say the voyage is too risky, while the crews are reported to have asked for danger money.

"It marks the start of a dangerous new phase in the nuclear industry," said environmental group Greenpeace.

It is the first time British merchant ships have been armed since World War II, and it is the first shipment of direct-use nuclear weapons material since 1992.

Departure imminent

The Pacific Pintail and Pacific Teal, currently docked in Barrow-in-Furness, Cumbria, are carrying reprocessed plutonium and uranium and could leave as early as next week.

BNFL have tried to keep the cannon under wraps

Each ship has been fitted with three 30mm cannon at a total cost of £8m. It will avoid the need for an even more expensive naval escort.

BBC correspondent Tom Heal says the ship's crew, who wanted an escort, are asking for danger money to compensate for what they say is the increased risk.

Simon Boxer, of Greenpeace International, says the voyage could face severe problems.

"You could have rogue states, perhaps even terrorist groups, who might be interested in this shipment.

'Safety risks'

"There are obviously the safety risks with transporting half a ton of plutonium with seven tons of munitions on each of the freighters."

The Oxford Research Group think-tank has also expressed concern.

Their spokesman Frank Barnaby said: "The security provided is totally inadequte for transporting about half a tonne of plutonium halfway round the world."

British Nuclear Fuels Ltd (BNFL) said security fully satisfied a US-Japan nuclear pact, and the UK authorities were also happy with the arrangement.

BNFL's Head of Transport Alasdair Thomas told the BBC the armaments conformed to regulations but would not say whether it made the ships safer than with a naval escort.

Officers from the UK Atomic Energy Agency Police will man the guns.

If the voyage proves uneventful, at least 80 plutonium shipments will take place over the next decade.

The exact route of the ship will remain a secret to prevent an attack by pirates or "rogue countries" which may want its nuclear bounty.