Greenpeace campaigners patrolling in the English Channel say they are still planning to intercept a shipment of plutonium en route to France.
The Pacific Pintail is one of the two cargo ships being used
On Monday afternoon the environmentalists had yet to catch sight of the two British-owned vessels.
Protesters had expected the ships to reach Cherbourg on Saturday.
French police say they have arrested five people, including a man from Guernsey, for protesting against the shipment of radio-active material.
The environmental group's ship, MV Esperanza, is anchored due south of Weymouth, Dorset.
Campaigning protesters now think the Pacific Pintail and the Pacific Teal will arrive on Monday night.
Yachtsman Eugene Riguidel and Greenpeace activist Jonathan Castle, from Guernsey, were among those arrested, said Greenpeace.
Nuclear Free Seas Flotilla spokesman Philippe Marechal told BBC News Online there was a woman among the five people arrested.
Mr Riguidel was arrested for sailing too close to the military zone
The weapons-grade plutonium is being transported on behalf of the US Department of Energy by two UK-registered vessels from shipping company Pacific Nuclear Transport Limited (PNTL).
British Nuclear Fuels Limited, PNTL's main shareholder, is refusing to reveal the ships' location.
The cargo of 125kg of plutonium, enough to make about 40 nuclear weapons, is heading from the US for France, where it will be converted into nuclear reactor fuel rods.
Greenpeace claims the cargo is dangerous and should not pass so close to UK shores. It also believes it could be a terrorist target.
A BNFL spokesman said that PNTL had carried over 170 shipments for a total of about five million miles without any incidents.
He refused to comment on security arrangements, but it has been reported that the ships have double hulls and are each guarded by 13 commandos and armed with a 30mm cannon.
He added: "The shipments are on schedule and everything is running to plan and there have been no safety and security issues."
The plutonium carriers could pass about 16 miles off the Lizard in Cornwall and Greenpeace campaigners are waiting between Guernsey and Start Point.
'Boiling' issue in France
Greenpeace spokesman Shaun Burnie said French environmental groups are also planning road blockades to prevent the cargo being transported to processing plants.
"It's boiling up as a real issue here in France," Mr Burnie said.
The Pacific Teal and the Pacific Pintail left the US a week ago on their trans-Atlantic crossing.
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 Cadarache in the south of the country to be processed and converted into mixed oxide nuclear fuel.