Film footage showing a controversial Cold War experiment exposing animals to plague bombs has been revealed.
The trials were conducted by scientists from Porton Down in Wiltshire using thousands of guinea pigs and monkeys.
The 1952 top secret film was obtained by an amateur historian from Dorset using the Freedom of Information Act.
The Defence, Science and Technology Laboratory said animal experiments had helped to drastically cut down "casualty statistics" at war zones.
Codenamed Operation Cauldron, the tests were conducted from a pontoon moored off the Scottish coast. The biological weapons testing on animals ended later in the 1950s.
The benefits of the research were judged to outweigh the risk of the biological agents being blown onshore.
About 3,500 monkeys and guinea pigs were killed after exposure to clouds of bacteria released by a small bomb.
Amateur historian Mike Kenner told the BBC programme Inside Out West that he had become aware of the footage after researching details of Operation Cauldron when it became declassified.
He obtained the film after a three-year campaign of repeated letter-writing using the Freedom of Information Act.
"So many animals were killed that they had to set up an assembly line of scientists," he said.
"It's more like looking at a fish-processing factory with people just filleting fish and it does become quite gory".
Biological warfare expert Dr Brian Barmer, who has written extensively about Operation Cauldron, said the footage "captures a particular moment in the Cold War where biological weapons were certainly on a par with atomic weapons in British defence policy".
"Getting hold of this film now, you forget quite how top secret this area was at the time. Really, none of this was known about," he said.
Animal rights campaign group the British Union for the Abolition of Vivisection (BUAV) said the Defence, Science and Technology Laboratory at Porton Down was still experimenting on animals today.
Last year, 8,168 animals were used in research, including 35 pigs for studies using explosives, it said.
BUAV chief executive Michelle Thew said: "This sort of cruel and grotesque research behind closed doors simply has no place in a modern society."
A statement from the Defence, Science and Technology Laboratory said: "As a direct result of the research undertaken by DSTL that involves animals at its Porton Down site, vast improvements have been made to the physical protection and the medical care received by UK armed forces in Afghanistan, and other theatres of conflict.
"Many men and women have been able to return from the front line to their families, rather than being mere casualty statistics.
"This work - which represents less than half a per cent of all UK research conducted on animals - undergoes a full ethical review including an assessment of the balance between benefit to man and the welfare of the animals.
"DSTL does not recognise today any of the practices shown in the 1952 film footage."
Inside Out West is on BBC One at 1930 BST on Monday 18 October.