Britain has agreed to pay compensation to three servicemen given the mind-altering drug LSD during tests in the 1950s, the government said on Friday.

The UK's Secret Intelligence Service, better known as MI6, commissioned the experiments fearing that the Soviet Union was developing a secret substance to brainwash its enemies and force prisoners to make confessions with a truth drug.

Don Webb, one of the volunteers who received an extra week's pay for the tests, said he and a colleague began to hallucinate after being given a clear liquid to drink.

"His face melted and opened so that I could swear that I could see his skull beneath the skin," he told BBC radio.