(CNN)The two police officers trailed the university professor as he left his home in the southwest London suburbs and walked to the local railway station.
His name was Eric Burhop. An Australian immigrant who had become one of the United Kingdom's leading nuclear and theoretical physicists, he was also a former member of the team that built the first atomic bomb, a prominent peace campaigner, and the subject of surveillance by security services on at least three continents in the 1950s.
Tall and well built, with thinning hair and a ruddy complexion, Burhop walked "with a slight stoop and takes noticeably short strides," a later report noted. "He usually carries a small brown attache case and raincoat. Wears herring-bone tweed sports coats and grey flannels, brown shoes."
The officers, members of the UK's national security-focused Special Branch, followed as Burhop took the train to London's Waterloo station, where he was joined by a man of "medium build, oval face, clean shaven, tanned complexion," with whom he proceeded to University College London.
They watched Burhop all day, as he went about his academic duties, had lunch in the university canteen, visited a local bank, and bought the evening newspaper. Due to the size of the campus, they missed him going home for the evening, expressing frustration in a report that they had "insufficient personnel available to cover all the exits."
In addition to the close surveillance of Burhop's activities -- one unlucky officer sat and watched him plant flowers in the garden of his house for several hours -- documents show his letters and telegrams were intercepted, his phone was tapped, and his friends and acquaintances investigated.
The surveillance, and a growing suspicion by the UK authorities that he had at least at one point been a spy for the Soviet Union, is documented in newly discovered files in the British archives.
"This is a curious case of an (alleged spy) who was allowed to hide in plain sight in London," said Susanne Roff, a UK-based researcher of nuclear history. Roff discovered the files in the British archives while researching the UK's nuclear tests in Australia.
Burhop died in 1980. The documents show he was never brought in for questioning by the British security services, nor was he arrested for his alleged spying. He may have been unaware that he was under surveillance at all.
His story touches on the deep paranoia in the early decades of the Cold War, and the numerous intelligence failures by the British security services who, on discovering another alleged spy within their jurisdiction -- following multiple embarrassing defections -- appear to have decided it was easier to leave him be rather than draw attention to their own potential failings.
CNN has reviewed the documents provided by Roff, as well as additional -- since declassified -- secret surveillance records on Burhop found by CNN in the British and Australian archives. Two independent academics also reviewed the documents on CNN's request. Their conclusions are presented below.
In a statement, Burhop's children pointed out that he was never publicly accused of being a spy for the Soviet Union or any other country, nor charged with any crime. They expressed concern that this article would mar the memory of "a hugely respected man of the highest integrity" who devoted his life to science and peace.
"He was a man of peace who worked tirelessly towards a nuclear bomb-free world," the statement said. "(We) would hate to see all that he achieved being hugely diminished by suggestions that he was a spy."
At 7:00 a.m. on August 29, 1949, a mushroom cloud bloomed over a remote part of the Kazakh steppe.
The event, which was held in secret, changed the world.
"We have evidence that within recent weeks an atomic explosion occurred in the USSR," President Harry Truman said in a statement to the American people about a month after the detonation.
"Ever since atomic energy was first released by man, the eventual development of this new force by other nations was to be expected. This probability has always been taken into account by us."
While US officials may have always anticipated the Soviets would join the nuclear club at some point, the speed at which they did came as a great shock. Months before the Kazakhstan experiment, American intelligence agencies were still predicting mid-1953 as the "most probable date" for a Soviet bomb.
The successful first test by the Soviet Union, and the end of Washington's nuclear monopoly, led to a massive increase in military spending in the US as the Cold War escalated.
The test also served to reinforce Washington's earlier decision to exclude its allies from all future nuclear research. Three years before the Soviet test, the Atomic Energy Act -- known as the McMahon Act -- was passed in the US. Outwardly, the act was intended to formalize civilian control of the US' nuclear industry. But it had a second purpose: tightening security.
Under the act, all information concerning the development and manufacturing of nuclear weapons was reclassified as "restricted data," meaning it could no longer be shared with allies.
These security concerns appeared to have been thoroughly vindicated when, months after the first Soviet test, it emerged that a British scientist on the team which developed the US bomb, Klaus Fuchs, had been leaking information to Moscow throughout World War II and after, according to the British security services.
Other spy scandals would soon follow, with senior members of the British Foreign Office and spy agency MI6 revealed to be secret Soviet agents.
At the start of the 1940s, as various powers raced to be the first to complete a then still theoretical atomic weapon, the UK's nuclear program was ahead of the US, with British scientists making key discoveries into how uranium fission could be used to create a powerful bomb.
Following Washington's entry into World War II in late 1941 after the Pearl Harbor attack, funding for nuclear research ramped up, and it soon became apparent that a joint program would more quickly deliver a bomb that researchers thought could potentially end the war.
In August 1943, leaders Winston Churchill and Franklin Roosevelt signed -- in secret -- the Quebec Agreement, which agreed the project would be "more speedily achieved if all available British and American brains and resources are pooled."
A number of prominent British scientists went to work with J. Robert Oppenheimer's team at Los Alamos. They included two naturalized UK citizens, German Klaus Fuchs and Australian Eric Burhop.
A member of the German Communist Party, Fuchs had fled to the UK in 1933 as the Nazis cracked down on their left-wing opponents. He earned a doctorate in physics from Edinburgh University, and in 1941 joined the UK-based precursor to the Manhattan Project, according to Britain's MI5.
Fuchs, through another exiled German Communist, got in touch with Soviet military intelligence and passed on secret documents on the atomic program. At Los Alamos, Fuchs had a "key role in the project over the next three years, developing many of the designs, equations and techniques used to build the first atomic bombs " -- information which he passed to his Soviet handlers, according to MI5.
"When combined with information from other sources, this helped the Russians to make rapid progress in developing what was effectively a copy of the American atomic bomb design," MI5 said in a report on Fuchs.
Another scientist on the British nuclear program, Italian Bruno Pontecorvo, was also later found to be sharing information with Moscow. Pontecorvo defected to the USSR in 1950.
While the US had its own problems with spies -- such as Manhattan Project member Theodore Hall, and the Rosenbergs -- officials in Los Alamos and Washington were furious with the British over the leaks, and regarded the UK as amateurish when it came to counter-intelligence, Roff said. The defection of MI6 agent Kim Philby in 1963 and the exposure of the Cambridge Five spy ring, a group of Soviet agents in the British government, would do little to change this opinion.
"The Americans were highly critical even though they had their own spy problems. They thought the British were very, very weak on security," Roff added.
Following the war, Burhop continued his work at the top levels of theoretical physics, and went on to be a leading peace and anti-nuclear campaigner, winning multiple international awards for his activism.