Russian spy in the Cotswolds who sent her reports back to Moscow via her washing line

ON A a hot summer’s day in wartime, a dapper young woman with a broad smile pushes a bicycle along a quiet lane deep in the Oxfordshire countryside. With her is a nervous-looking man with intense brown eyes. Passers-by notice the couple holding hands.

Yet far from being innocent star-crossed lovers, they are two of the Cold War’s most important spies, serving Stalin’s military intelligence network and betraying some of our vital nuclear secrets to Moscow.

The ‘romance’ is a cover for controller and agent.

The woman is Sonya, the code name of Ursula Kuczynski, of the GRU (Soviet military intelligence) – a German-born illegal living undercover with her British-born husband Len, also a Soviet spy. She sends secrets back to Moscow with the help of a specially adapted washing line.

The man is Klaus Fuchs, also a German immigrant. He is a gifted physicist, working on the Tube Alloys project.

At the heart of it is the manufacture of pure uranium-235. Tube Alloys would go on to become the basis of the Manhattan Project in Los Alamos, New Mexico, which developed the first nuclear weapon. Its betrayal by Fuchs gave Moscow a leap forward worth at least two years in its nuclear programme.

New material unearthed from MI5 and East German files confirms that both before and after the war, many of Britain’s nuclear secrets were betrayed by Fuchs, with Sonya as his controller.

Worse still for the reputation of Britain’s security services, recently opened files reveal that MI5 had been alerted to the dangers posed by Sonya and her brother Jurgen, but failed to act.

Taking centre place in the conspiracy was the Isokon – a fashionable, white modernist building in Hampstead, North London, also known as the Lawn Road flats. It is still impressive – a large ocean liner erected among the Victorian terraces as a ‘utopian design’ for a new generation of idealists.

Residence there brought liberation from ‘tiresome domestic tasks’, with an on-site restaurant,

MI5 was told about the danger but failed to act

shoe-shining service and communal balconies. Long dinners were cooked by a celebrity chef. Prominent artists including Piet Mondrian, Ben Nicholson and Henry Moore gathered in the basement Isobar restaurant.

Had communism ever amounted to more than misery in practice, it would have looked a lot like the carefree life of the Isokon – a fact reflected in a good deal of the high-minded conversation of those who gathered there. For as the recently opened MI5 files make clear, the flats attracted not just communist intellectuals, but Soviet agents.

In the period between the end of the 1930s and 1950, more than 20 agents and informers were