On a scrap of blue paper, unsigned but in the distinctive handwriting of one of Britain's leading scientists, four words reveal one of the most momentous events in the country's history. 'Looks just about OK.'

The note was written by William Penney, director of the atomic research centre at Aldermaston. He was writing about Britain's secret post-war project: the development of the hydrogen bomb, a weapon with the ability to destroy whole cities and millions of people in a flash of searing heat and light.

Penney had just worked out how to make the weapon detonate successfully. It was September 1955.

The note, and the remarkable story of Britain's nuclear weapons project, is revealed in the first officially sanctioned history of the 'superbomb project'. The book, which had to be given clearance by the Cabinet Office because the details it contains are so sensitive, has taken nearly 10 years to produce and is written by Lorna Arnold, a former departmental record officer with the Atomic Energy Authority. She was given special access to the closed files at Aldermaston which will not be handed to the Public Records Office for decades to come.

The book, Britain and the H-Bomb , to be published this week, reveals that the four men behind Britain's nuclear deterrent believed they were ambassadors for world peace, despite inventing one of the most destructive weapons known to man. Penney, along with physicists Keith Roberts and Brian Taylor, and William Cook, the deputy director of Aldermaston, thought that the full horror of the H-bomb was such that it would never be used.

'William Penney and William Cook had seen the appalling effects of years of conventional war,' Arnold said yesterday from her home in Oxford, where she has now retired. 'They believed that this weapon would mean the end of world wars.'

Arnold said she hoped the book would redress the balance of earlier histories of nuclear development which put America at the forefront. Although Robert Oppenheimer, the American scientist who ran the Los Alamos nuclear development project during the Second World War, is recognised across the globe, the names of Penney, Roberts, Taylor and Cook are unknown outside the rarefied world of nuclear science.

'None of them ever publicised their role or wrote memoirs - they had a great belief that they were simply part of a larger effort,' Arnold said. 'But they were dealing with one of the most complicated and difficult scientific challenges, and they were successful.'

Arnold, who is now 85, said the book became a 'labour of love', She began work at the Atomic Energy Authority in the 1950s, an era when the future of nuclear science held out the twin possibilities of total destruction or limitless electrical energy. At that time the nuclear industry was employing 27,000 people.

The book shows how the men who researched the H-bomb were at the forefront of theoretical science and often had to rely as much on their imagination as their practical knowledge. John Dolphin, who was head of the 'project committee' at Aldermaston, admitted his 'complete ignorance' of physics, yet he 'produced elegant drawings of strange "thermonuclear" devices - perhaps his own ingenious ideas,' Arnold writes.

As they grappled with what is widely believed to be the most significant discovery of the twentieth century - the process of nuclear fusion - the scientists often lost heart. At a meeting in December 1955 the main scientists met to pull together their ideas. 'Does anyone know how it is done?' Cook said. There was an 'embarrassed silence', Arnold says, before revealing that the moral dilemmas and problems with resources faced by the scientists almost led to the breakdown of the whole project.

Such was the exasperation at the slow progress and complicated nature of the work, Cook sent out worried instructions to his staff to 'keep it simple, stupid!'.

'The plain fact is that weapons work is unpopular and nobody wants to do it,' Penney wrote in a memo in 1954 in which he admitted to being 'terribly depressed'. He continued: 'If I have to run with just the same people that I have had for the last year or two we are going to make a mess. Even the programme which is definite is above the present capacity of the establishment to bear.'

The project was shrouded in the utmost secrecy because of fears that both the US and Russia, which were developing their own H-bombs, would seek to exclude Britain from the superpower league that was rapidly developing. The Government kept as much as much as possible from the public after concerns were raised that mass marches by the nascent Campaign for Nuclear Disarmament could derail the research.

Arnold said that many of the most important and sensitive documents were destroyed by Aldermaston during a 'reorganisation' in the 1960s.

The book reveals the huge tensions being created by the Cold War. In 1954 a committee under the command of the Joint Chiefs of Staff, the leading military body in Britain, said they considered that the US could 'deliberately precipitate war with the USSR in the near future'. Any conflict would lead to the 'total destruction' of the UK, which they believed was Russia's primary target - just nine years after the two countries had been allies against Nazi Germany.

In the same year Penney, who was put under intense pressure to 'get results', wrote a report for Winston Churchill, the Prime Minister, in which he revealed the destructive capacity of the weapon. 'He described the effects of a five-megaton "true" H-bomb dropped on London,' Arnold writes. 'It would produce a fireball two miles across and a crater three-quarters of a mile wide and 150 feet deep. The Admiralty Citadel (Whitehall's emergency nerve centre) would be crushed at a distance of one mile, houses would be wrecked three miles away and badly damaged at seven miles; within a radius of two miles all habitations would catch fire.'