She was dubbed the Spy Who Came In From The Co-Op but for years Melita Norwood's neighbours regarded her as an ordinary, if eccentric, widow.

Aged 87 by the time she was unmasked as a former Soviet spy in 1999, Norwood would drink her tea from a Che Guevara mug and deliver copies of the communist daily newspaper, the Morning Star, to friends and neighbours of her home Bexleyheath, south east London.

But while her political leanings were clear, few would have suspected Norwood - codenamed Hola - was the most important female agent ever recruited by the USSR.

It was only when MI5 recruited Professor Christopher Andrew, an intelligence historian, to study the Vasili Mitrokhin archive that her secret identity was made public.

Her name is included in two pages of notes contained within the newly released archive. Documents from the Mitrokhin Archive - described by the FBI as the most complete intelligence ever received from any source - have this week been opened to the public for the first time after being kept at a secret location for more than 20 years.

They outline how she was recruited in 1937 on an "ideological basis" following a tip from the leadership of the British Communist Party.

At the time Norwood, who died in 2005 aged 93, worked as a clerk, and then a secretary, for the British Non-Ferrous Metals Research Association in London.

The company was engaged in metallurgy research which was invaluable in Russian attempts to mass manufacture modern armaments.

Her Euston-based employer was deeply involved in the top-secret project associated with the building of Britain's atomic bomb, codenamed Tube Alloys.

Describing Norwood, the note continues: "Characterised as a loyal, trustworthy, disciplined agent, sought to bring maximum benefit.

"Passed on very large number of scientific and technical documentary materials that are practically applied in Soviet industry."

The note outlines how in 1968 she was awarded the Order Of The Red Banner Of Labour for her service to intelligence.

In 1962 she was awarded a lifetime pension, paid up to June 1973, of £20 a month in recognition of her "many years of excellent work".

However, as the years went on, her value as a spy dwindled.

In 1974 Soviet agents re-established contact but judged that she note longer had "intelligence capabilities" so operational contact was not maintained, the notes say.

It adds that five years later, Norwood and her husband visited the Soviet Union.

"(She) refused money, saying she did not need it, did not see the point of having meetings, since she had no intelligence capabilities," he reads.

The mother of one, who was married to a maths teacher, was born near Southampton to an English mother and Latvian father and joined the Communist Party in 1936.

In 1965, seven years after her retirement, security services in the UK became aware that she was a security risk.

But it was not until the Mitrokhin archive emerged that she was publicly outed.

She told the press: "I never considered myself a spy, but it's for others to judge," before politely closing her front door to the media.

Prof Andrew said there was evidence within the archive that Norwood was a highly prized Soviet asset - valued above Kim Philby, who in 1945 rose to head of the British Secret Intelligence Service's anti-Soviet section while operating as a KGB agent.

The archive suggests that while the KGB was initially sceptical about Philby's value, agents were actively pursuing Norwood during the 1930s.

"At that time, security agencies were somewhat chauvinistic so to prefer a woman to Philby at that time, shows just how important she was to them," Prof Andrew said.

"It is unlikely she realised the significance of much of what she was passing on - to a non-expert and stripped of its context, it would have seemed relatively mundane.

"But there is no doubt from these documents that the information she passed on were extremely useful to the KGB."

Norwood was never prosecuted as such a move was deemed "inappropriate" by the attorney general.

Vasili Mitrokhin was dressed in beggars' rags and carrying a suitcase full of dirty underwear, so British embassy staff could have been forgiven for dismissing him as a crank when he arrived on their doorstep offering access to top-secret Soviet files.

Instead, in a typical show of British hospitality, a young female diplomat offered the defector a cup of tea - thus securing one of the biggest intelligence coups in history.

Following the fall of the Soviet Union in 1992, the retired KGB archivist chose his moment to leave with a handful of classified documents detailing Russian "illegals" acting as spies abroad hidden in his suitcase.

He arrived in an unnamed Baltic city, often reported to be Riga in Latvia, and offered his services to the West.

Some accounts say that Central Intelligence Agency (CIA) officers at the US embassy did not consider him credible.

Professor Andrew said he was deterred by the long queues outside the embassy.

Whatever the reason, he then made his way to the British embassy where he was soon made welcome.

"Once there, he opened his suitcase, revealing the documents along with his dirty underpants and food he had packed for the journey and asked to speak to somebody in authority," Prof Andrew said.

Once the value of his offer became clear, he and his family were exfiltrated to Britain and the documents helped secret services establish the extent of intelligence gathered by the Soviets throughout the cold war.

A further 25,000 pages of files, covering operations dating back to the 1930s, were later retrieved from his home.

Between 1972 and 1984, his duties with the KGB had included overseeing the transfer of the agency's archive to a new location, meaning he had unlimited access to thousands of files from a global network of spies and intelligence gathering operations.

Disenchanted with the Soviet regime, he began making copies and extraordinarily detailed notes

At first he would screw his notes into tiny balls to smuggle them out in his shoe but soon realised that his position meant he could not be searched so he could simply slip them into his pocket.

Prof Andrew said: "I think his motivation was pretty simple.

"We're all accustomed to the idea that there were genuine dissidents who were prepared to take extraordinary risks.

"They were all hoping that following de-Stalinisation, socialism with a human face would emerge.

"When that didn't happen, they could either stand up and be sacked or undermine the system from within.

"I think the risks he took reflect the genuine beliefs he held."

Mitrokhin died from pneumonia aged 81 in 2004.