I can prove how British scientists really saved America's Manhattan Project from disaster

It was two years ago that the Oscar-winning blockbuster Oppenheimer sent the message that it was the Americans, or rather one American in particular, who was responsible for the atomic explosion that brought the Second World War to an explosive close.

The man in question was native New Yorker Robert Oppenheimer, scientific director of the Manhattan Project's top-secret programme of nuclear bomb-making deep in the New Mexican desert.

By August 1945 "Little Boy" and "Fat Man" were ready to be loaded on to B-29 bombers, flown halfway across the world and dropped on two of Japan's major cities to devastating effect.

Not so fast, writes Gareth Williams. In this myth-busting book, he reveals that, without the involvement of British physicists at an early stage, the all-American "Manhattan Project" would never have got off the ground.

And the consequences of that would have been catastrophic, leaving the USA and most of Europe at the mercy of German scientists who were rushing to develop their own weapons of mass destruction.

In which case it could so easily have been London or Washington rather than Hiroshima and Nagasaki that were engulfed in a giant mushroom cloud.

Williams is a retired university professor and, as he explains, a member of CND, who was first alerted to Britain's forgotten contribution when he encountered declassified papers that referred to a mysterious "MAUD committee".

Digging further he discovered that MAUD had been set up in 1940 to investigate the feasibility of Britain producing an atomic bomb in the immediate future.

The nudge had come from a memo written by two German-Jewish physicists who had escaped from Nazi Germany and were now working at the University of Birmingham.

In their document, Otto Frisch and Rudolf Peierls hypothesized that by using an isotope called Uranium-235 they would be able to build a 5 kg bomb with the explosive power of thousands of tons of TNT.

Ironically as "enemy aliens" the pair were not allowed to sit on MAUD, which was staffed by British scientists, including several Nobel Laureates.

Even so, it didn't take long for the committee to excitedly confirm their findings.

From now on it was all systems go on a project that would henceforth be known by the deliberately dull name of "The Tube Alloys Directorate".

Initially Prime Minister Winston Churchill was insistent that Britain should go it alone.

At this stage the Americans were dragging their feet on the nuclear option, complacently believing that their stockpile of conventional weapons would be enough to defeat the Germans.

This was despite Albert Einstein, the world's pre-eminent physicist, writing urgently to President Roosevelt as early as 1939 to warn that America needed to throw everything behind the atomic bomb if it were to have a hope of beating the Germans to the finish line.

Just in time, the Americans came to their senses and started to play catchup.

Simultaneously, British scientists were realising that, while they had the theoretical knowhow, they lacked the material resources to build a bomb: plutonium and uranium were astronomically expensive.

Eventually, the two countries agreed to co-operate, and 84 British scientists made the dangerous journey across the Atlantic to turbo charge the Manhattan Project. The rest, as they say, is History.

Gareth Williams understands every detail of the science, from "heavy water" to "cyclotron" by way of "gaseous diffusion".

There's a helpful cheat sheet of terms in case you get stuck but, luckily, Williams writes with the clarity and pace of John Le Carré.

So even the most scientific dullard will be able to follow along. Strip out the science and what you are left with is a thrilling human drama, told in pacey chapters like "Whoddunit" and "On the Run".

It's all underpinned by the terrifying sense of how different Britain's fate might have been if it hadn't been for what Williams stirringly describes as "the most significant international collaboration of the 20th Century."

The Impossible Bomb: The Hidden History of British Scientists and the Race to Create an Atomic Weapon, by Gareth Williams, is published by Yale University Press.