A sinister Russian plot to destroy the city of London using ‘very powerful’ nuclear bombs has been revealed.

British atomic expert William Penney, who helped develop the UK’s first atomic bomb, exposed the destructive scheme in a top-secret letter sent in 1954.

He warned the Atomic Energy Authority chairman Edwin Plowden that the Russians were prepared to launch the devastating assault on the capital city.

Sinister: British atomic expert William Penney revealed a terrifying Russian plot to launch a nuclear airstrike on London, during the Cold War

Devastation: Penney warned in his handwritten letter that the 'very powerful' bombs would have been more destructive than that dropped on Japan during the Second World War. Pictured, the aftermath of the atomic bomb dropped on Hiroshima in 1945

‘Instead of using, say, 32 bombs on London, they would probably use three, four or five very powerful ones which would give the same total damage but need not be aimed accurately,’ he wrote.

The series of earth-shattering explosions would have caused ‘complete destruction’ for three miles in every direction from the centre of the blast, covering an area of nearly 30 miles.

Penney, whose handwritten letter has been released by the National Archives for the first time, particularly highlighted Croydon, in south London, Uxbridge in the west and Romford in the east as being under threat.

But, he continued, all British people would suffer from the catastrophic nuclear fallout in the wake of the Cold War attack, reported the FT.

Warning: The top-secret letter warning of the planned attack came from British atomic expert William Penney (left, pictured with his inquiry team Professor J.M. Kay, Mr D. Peirson, Dr B. Schonland and Professor Jack Diamond)

Fallout: Penney said in his letter that although London was the target, all British people would suffer from the catastrophic effect of the nuclear strike. Pictured, the devastation after the 'Fat Man' nuclear bomb was detonated over Nagasaki in August 1945

Enemies: The series of earth-shattering explosions in London would have caused ‘complete destruction’ for three miles in every direction from the centre of the blast. Pictured, British army tanks in 1954

The historic letter warned the bombs that the Russians planned to use would have been more powerful than that used on Nagasaki, in Japan, by the U.S. during the Second World War, on August 9, 1945.

At least 74,000 people were killed as a result of that devastating blast, and a further 75,000 were wounded.

A note from the Chairman of the Home Defence Committee in March 1955, also released by the National Archives, added: ‘We cannot tell with certainty how much warning we should get before an attack was made on the United Kingdom.

‘But we consider that, where essential for planning purposes, departments may continue to proceed on the assumption that the government would be able to detect a deterioration in the international situation some six months before the war came and we would know, say, seven days in advance that an attack on this country was to be expected.’

The U.S. also bombed the Japanese city of Hiroshima just three days before Nagasaki, killing 140,000. The twin attacks were the first and only time atomic bombs have been used on a civilian population.

Earth-shattering: At least 74,000 people were killed as a result of the devastating blast over Nagasaki in 1945, and a further 75,000 were wounded. Pictured, the deserted streets of Cornhill in London, in 1954

Historic: Penney's handwritten letter has been released by the National Archives for the first time. Pictured, a British army tank in 1954

A five-tonne plutonium bomb, known as the ‘Fat Man’, exploded 500m above the thriving port town, on the western side on the Japanese island of Kyushu.

Wall clocks that stopped working at 11.02am, when the explosion occurred some 70 years ago, are displayed at the Nagasaki Atomic Bomb Museum.

Japan’s surrender on August 15, 1945, brought an end to the Second World War.

Penney died in 1991, aged 81.

The emergence of the Cold War plot comes as the UK’s nuclear policy returns to the focus of national debate.

Potential victims: Penney warned the Atomic Energy Authority chairman Edwin Plowden that the Russians were prepared to launch the devastating assault on the capital city. Pictured, crowds in White City, London, in 1954

Defence secretary Michael Fallon warned on Wednesday of ‘worrying challenges to the nuclear consensus’ that would endanger national security.

His speech came in response to Labour leader Jeremy Corbyn’s strongly anti-nuclear stance.