Russia planned to drop "very powerful" atomic bombs over London that could have destroyed the British capital at the height of the Cold War, top-secret letters made public for the first time revealed.

The astounding revelation was made in a top-secret letter sent in 1954 by the late British atomic expert William Penney, who helped develop the UK's first atomic bomb, the Daily Mail reported.

In the letter, Penney warned the Atomic Energy Authority chairman Edwin Plowden of Russia's terrifying plan to launch the nuclear attack on London, which could have caused "complete destruction" over a three-mile radius if it pushed through.

"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," Penney said in his letter, as quoted by The Daily Mail.

The British atomic expert, who died in 1991 at the age of 81, also mentioned in his handwritten letter that Russia wanted to use nuclear bombs more powerful that the ones dropped by the United States on Nagasaki in Japan during World War II, on Aug. 9, 1945.

The letter also identified particular areas in London that were most vulnerable to the planned Russian nuclear attack: Croydon, in south London, Uxbridge in the west and Romford in the east.

Penney also warned that a global war involving the use of nuclear weapons "would threaten life throughout the world."

If Russia's nuclear plan pushed through, at least 74,000 people in London would have been killed, and 75,000 other individuals would have been injured in the devastating blast, The Daily Mail said.

Not only that: generations of British people would have suffered from the catastrophic nuclear fallout for years, it added.

The National Archives released Penney's historic letter this week. It also released a note from the Chairman of the Home Defence Committee in March 1955, which stated: "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 official added in his note.