A shipment of weapons-grade plutonium being transported to France from the USA will be passing within 26km (16 miles) of Cornwall on Friday.
The Pacific Pintail is one of the two cargo ships being used
Earlier this week people living in South West England and the Channel Islands were told that precautions to protect the material were in place.
The controversial shipment will also be passing within 50km (31 miles) of the Channel Islands en route to Cherbourg.
Shipment authorities say the vessels are armed and safe to carry the waste.
The shipment is of 140kg of weapons-grade plutonium, enough to make 40 nuclear bombs.
It is being transported on behalf of the US Department of Energy by two UK-registered vessels from shipping company Pacific Nuclear Transport Limited
(PNTL), whose main shareholder is British Nuclear Fuel Limited (BNFL).
The Pacific Teal and the Pacific Pintail left the US a week ago on their trans-Atlantic crossing. They should enter the English Channel by the weekend.
The highly radioactive material has been taken directly from nuclear warheads, following disarmament agreements with Russia.
Once in France, the plutonium will be transported to the south of the country to be processed and converted into mixed oxide (Mox) nuclear fuel.
Campaign group Greenpeace claims the vessels pose a potential "environmental and terrorist threat" and former UK Atomic Weapons Establishment scientist Dr Frank Barnaby said an on-board accident could have serious consequences.
Dr Barnaby also criticised the level of security being provided for the shipment by the company responsible for the transportation, Cogema-Areva Logistics.
He said a warship should have been used to transport the nuclear material because of the risk of a terror attack.
His view is shared by Torbay's MP, Liberal Democrat Adrian Sanders.
Mr Sanders said: "My concern is that this particular substance should be dealt with in the USA and turned into the fuel there.
"It shouldn't be transported half-way across the world at any time, let alone in this heightened period of terrorist activity."
Shaun Burnie, from Greenpeace, said the shipment "did not need to happen".
He added: "This is bomb material that cannot be, and should not be, treated as if you're just handling bananas or something."
West Cornwall MP Andrew George has written to the Home Secretary David Blunkett, asking for reassurance over safety.
Cogema-Areva Logistics said fears were unjustified. It said the vessels had a squad of armed officers from the UK Atomic Energy Agency Constabulary and were also carrying naval cannons, had satellite monitoring and heavily reinforced hulls, plus back-up engines.
A BNFL spokesman said PNTL had carried over 170 shipments for a total of about five million miles without any incidents.
He added: "PNTL have got a huge experience of carrying all sorts of nuclear materials over the last 30 years.
"These things don't just happen on a whim. They are carried out under very strict regulatory authorisations."