The chilling document that traces nuclear weapons back to Britain - and the threat we now face

Eighty years ago today, an American B-29 bomber dropped an atomic bomb on the Japanese city of Hiroshima.

It was the dawn of the atomic age, but the birth of the bomb can be traced beyond the deserts of New Mexico to Britain, five years earlier. A copy of a hand-typed document, now in the Bodleian library in Oxford, is the first description of an atom bomb small enough to use as a weapon.

The Frisch-Peierls Memorandum was written by two nuclear physicists at the University of Birmingham in 1940. Otto Frisch and Rudolf Peierls don't feature in the film Oppenheimer, but their paper is credited with jump-starting the Manhattan Project that ultimately built the bomb.

Both Jewish scientists who had both fled Nazi Germany, they built on the latest understanding of uranium fission and nuclear chain reactions, to propose a bomb made from enriched uranium that was compact enough to be carried by an aircraft. The document, so secret at the time only one copy was made, makes for chilling reading.

Not only does it detail how to build a bomb, but foretells the previously unimaginable power of its blast. "Such an explosion would destroy life in a wide area," they wrote.

"The size of this area is difficult to estimate, but it will probably cover the centre of a big city." Radioactive fallout would be inevitable "and even for days after the explosion any person entering the affected area will be killed". Both lethal properties of the bombs that would subsequently fall on Hiroshima and Nagasaki, killing around 100,000 instantly and more than 100,000 others in the years that followed - most of them civilians.

Read more:Hiroshima survivor's stark warningThe 'destroyer of worlds' who built the atomic bomb 'The most terrifying weapons ever created' Those bombs had the explosive power of around 16 and 20 kilotonnes of TNT respectively - a force great enough to end the Second World War. But compared to nuclear weapons of today, they were tiny.

"What we would now term as low yield nuclear weapons," said Alexandra Bell, president of the Bulletin of Atomic Scientists, which campaigns for nuclear disarmament. "We're talking about city destroyers…these really are the most terrifying weapons ever created." Many of these "high yield" nuclear weapons are thermonuclear designs first tested in the 1950s.

They use the power of nuclear fission that destroyed Hiroshima to harness yet more energy by fusing other atoms together. Codenamed "Mike.