The Metropolitan Police has begun an investigation into aspects of Britain's nuclear testing programme in the 1950s.

Thousands of veterans who took part in the tests in Australia, Christmas Island and other islands in the Pacific later complained of ill health - a large number have since died.

Almost no-one had any kind of protective clothing and many now blame the tests for making them ill.

The issue is already in the civil courts, but now Scotland Yard says it has begun a criminal investigation against the Ministry of Defence and some of the scientists involved.

Papers seen by the BBC suggest the inquiry has already been under way for three months.

It focuses on the case of an RAF squadron leader, Eric Denson, who fell ill and committed suicide years after allegedly being ordered to fly his aircraft unprotected through a highly radioactive mushroom cloud.

Mr Denson was healthy before the test in 1958 but then developed chronic illness which led to depression and eventual suicide in 1976.

Human guinea pig

Alan Care, the solicitor acting for Mr Denson's widow Shirley, told BBC Radio 4's Today programme: "The claim is that he was instructed by the Ministry of Defence to pilot an aircraft through a mushroom cloud and in effect was being used as a human guinea pig."

The police inquiry will investigate whether it was legal for military chiefs to order the air crew to fly through the cloud.

Mrs Denson alleges the government "knowingly and maliciously" exposed her husband to "deadly and legally prohibited levels of radiation which ultimately led to his death".

The MoD has denied that Mr Denison was used as a human guinea pig to test radiation, but said it was co-operating fully with the police.

Scotland Yard said on Thursday: "We can confirm that the Metropolitan Police Service specialist crime group are carrying out preliminary assessment of information received in August this year.

"We are not prepared to say who requested this assessment to be carried out or in connection with what."