FOR more than half a century it has remained one of the most closely guarded secrets of the cold war.

In a series of biological weapons tests off the Isle of Lewis, monkeys and guinea pigs were suspended from cages on pontoon bridges and exposed to clouds of deadly bubonic plague.

Now, newly declassified files have revealed for the first time the recklessness of Operation Cauldron, where lives were put at risk and government officials embarked on a cover-up to prevent the tests being exposed.

The documents reveal that the fishing boat Carella strayed into the test zone and the crew were put at risk from the tests.

However, ministers decided not to warn the crew and failed to prevent them returning to port, potentially putting the population of Britain in danger from the deadly pathogen.

Instead, they ordered two ships, including a destroyer, to trail the trawler on its three-week fishing trip to Iceland and to intercept any distress calls should a crew member fall sick.

One memo makes it clear that the priority was to cover up the incident rather than protect the lives of those on board. “Requirements gravely political are more important than what may seem best from a purely medical point of view for individuals,” read the instructions to the captain of the destroyer. A further document reveals that the decision was taken not to inform the secretary of state for Scotland or the islanders of Lewis.

The tests were part of a biological weapons research programme based at Porton Down conducted between the second world war and the mid-1950s. The Western Isles were chosen as a testing ground because of their remoteness and their sparse population.

In 1952, bubonic plague bacteria were carried to the area aboard Ben Lomond, a tanker converted into a floating bio-weapons laboratory. The vessel was anchored about 20 miles from the coast of Lewis.

On September 15, the Carella, from Fleetwood, near Blackpool, sailed into the exclusion zone. Detailed minutes reveal that orders were given to release the toxic gas two minutes after the trawler appeared on the scene.

Senior government figures decided that the crew should not be told, despite the risk to public health: “It would be clear to all that these precautions were being taken against bacteriological infection and the destruction of rats would suggest that this infection was plague,” concluded Lord Duncan Sandys, minister of supply, during one meeting. “It would not be possible to conceal the reason for this action. In my opinion it should be ignored.”

The documents also reveal that ministers mistimed the Carella’s return to harbour and failed to intercept the vessel before its crew returned to shore.

Dr Brian Balmer, a specialist in biological weapons from University College London, who has studied the documents for a new Radio 4 documentary on the incident, said the government had been motivated by cold war paranoia.

“This incident was subject to many layers of security,” he said. “Firstly, there would be extra security because these were military tests held during the cold war. Finally, they were biological weapons, which added yet more secrecy.

“The decision not to inform the crew would not have been taken on purely scientific grounds but would have been informed by security fears.”

Derek Bellerby, one of the Carella crew members, knew nothing about the incident until he was tracked down by the makers of the documentary.

“What about when we went onshore? Who’s to say that nobody would have contracted it?” said Bellerby. “It just beggars belief that something like this could happen.”

Councillor Angus Nicholson, head of the Western Isles environmental services committee, added: “The fact that these experiments were ever carried out off the coast of the Western Isles shows complete disregard for the population.

“The documents show an extremely patronising view of this area that the government held at the time. This does not reflect well on them or on any government that has kept the information secret.”

The documentary Operation Cauldron will be broadcast on Radio 4 on September 22 at 8pm

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