Croydon was targeted for nuclear annihilation in a Russian Cold War plot to wipe out London, Britain’s spies believed.

William Penney, a scientist who helped develop the UK’s atomic weaponry, warned in 1952 of the astonishing plans to obliterate the borough with a thermonuclear fireball.

Security officials' worries were revealed in documents released to the public for the first time by the National Archives this week.

The papers show Whitehall feared the Russians would target Britain as a key ally of the US, which detonated the first hydrogen bomb in the Marshall Islands in 1952.

Writing to Edwin Plowden, chairman of the Atomic Energy Authority, Mr Penny suggested the Russians would drop three to five nuclear bombs at sites surrounding London to destroy the city.

A diagram sketched in pencil in his notes identified Croydon, Uxbridge and Romford as potential targets of the strikes, according to the Financial Times.

Mr Penny wrote: "Instead of dropping 32 bombs on London, they would probably use three, four or five very powerful ones... with a radius of total destruction of two to three miles".

The papers reveal the British government had previously planned for a "survivable" nuclear attack involving 200 atomic warheads but not a H-bomb strike.

The government subsequently arranged to stockpile thorium and produce heavy water - both essential ingredients for an H-bomb - in a bid to achieve nuclear "equilibrium".