It was the cancelled dinner date that did it.

Araceli Gonzalez de Pujol was bored and desperately homesick. The attractive, 23-year-old Spanish woman was stuck in a house in north London with no social life. She had been away from Spain more than a year, missing her family, with no relief or respite from raising her two young children.

She spoke little English. Her 31-year-old husband worked late, and some weekends, on important business.

So when she met a Spanish family and they invited the couple to dinner at the Spanish Club, she was delighted.

Her husband said no. She picked a fight.

The fight escalated into one of the key – though rarely recognised as such – confrontations of World War II.

Her husband was no average inattentive spouse – as is made clear in his MI5 file, which was declassified this week at the National Archives.

The year was 1943 and he was well on the way to becoming history's greatest double agent.

By that summer Juan Pujol Garcia was, as far as the Nazis were concerned, running their entire spy network in Britain – a sophisticated and busy group who encoded letters in invisible ink with details of Britain's troop movements, defences and armaments. The network had even penetrated the Ministry of Information.

The Germans were so delighted with Juan's work they doubled his pay. They didn't bother sending any more spies over the Channel.

But in fact, every spy in his network was a work of fiction. Code-named '"Garbo", Juan was the jewel in the British intelligence crown.

He gave them insight into Axis intentions and knowledge, through the questions Berlin asked of him (as well as asking for reports on Britain's defences, domestic food stocks and shipbuilding, they asked him to research "what would be the result of Japanese aggression against Hong Kong, Singapore, the Dutch East Indies and Australia").

The Germans provided him with a new "unbreakable" cypher for their most secret communications – very useful to the code-cracking boffins at Bletchley Park – which one MI5 agent remarked, "is perhaps the strongest indication to date of the implicit trust [the Germans] have in Garbo … or of their incredible stupidity".

"[Garbo] puts a weapon of the highest value into our hands," another wrote in mid-1943. "[He] holds almost unlimited potentialities for good, especially in the sphere of deception."

The next year they would wield that weapon, crucially. He convinced the Germans that General George Patton had assembled a million-man army which would land at Calais.

Once D-Day began, as the Allies landed at Normandy, Garbo sent a long telegram insisting it was a feint – and Berlin believed him, weakening their counter-attack by keeping their Panzer divisions pinned at Calais until the Normandy beachhead was well-established, and the Nazi forces in retreat.

But a year earlier, on the home front, things were not going so well.

In February 1943, Garbo's MI5 handler wrote to his superiors: "Mrs Garbo has periodical fits of homesickness and a desire for a more active and adventurous life."

No wonder. To minimise the risk of alerting the Germans to Garbo's work, MI5 carefully restricted their social circle. Spaniards were a danger, because some might be Nazi sympathisers. Indeed, they were discouraged from meeting any foreigners.

And Garbo himself was not much fun, the MI5 file notes: "By the time he gets back from his work he is tired and has his mind on the case and is not always in the best of humour to chat with [his wife] about trivialities … her existence therefore is not a very enjoyable one".

"Her desire to return to her country and in particular to see her mother has driven her to behave at times as if she were unbalanced … she is a highly emotional and neurotic woman."

The cancelled dinner date was the last straw. Mrs Garbo and her husband had a furious row, after which she called his MI5 handler. In a series of melodramatic calls she demanded they send her back to Spain, to her family. And she threatened to expose his work to the enemy.

The calls were recorded by the phone tap on the house and transcribed for posterity in the Garbo file.

"I must leave England because I don't want to stay another day here," she said. "I don't want to live one more day more with my husband ... I don't want to be here and you don't take it into consideration and don't take any notice of me ... I'll take my revenge.

"If tomorrow at this time you don't give me the answer that I am to leave England, I am going to go to the Spanish Embassy and make a row … I am going to spoil everything … I know what I have to do to spoil the work."

The MI5 file even records that she then "slammed down the receiver".

Garbo and his handler had a crisis meeting. Garbo said he "was convinced" she would not carry out the threat, but didn't want to take responsibility if he was wrong.

The next day MI5 came back to Garbo with a plan: he would tell her MI5 had blacklisted him, meaning they could not leave the country, and he would pretend to get a new job at the BBC while continuing in espionage.

But Garbo didn't like the plan, his handler recorded: "[He] put forward a rather more drastic plan, which was subsequently put into operation."

It was a convoluted, brilliant and cruel scheme (MI5 later praised its "extraordinary ingenuity") to utterly destroy his wife's will.

First, they placed a watch on the Spanish Embassy.

Then a note was delivered to Mrs Garbo saying her husband had been arrested. As anticipated, she phoned his handler, who told her his superiors had confronted Garbo about his wife's threats of treachery, he had defended her, and they had decided to punish him.

Her pleas for clemency were ignored. They sent round agent Haines to her house, whose handwritten statement sits in the Garbo file. He arrived about 8pm to "find her sitting in the kitchen with all the gas taps turned on [and] the door of the children's bedroom carefully shut".

She told him to leave but "she worked herself into several hysterical outbursts in the course of the evening", again threatening suicide.

Haines said there was a "90 per cent chance this was mere play-acting" and indeed Mrs Garbo later said she had thought she was being framed.

But the handler sent his wife over, who Mrs Garbo considered a friend, and over the night they washed and ironed Garbo's clothes in preparation for "a long period of detention".

Finally, Mrs Garbo was convinced. By the next morning, Haines wrote, she was "nervous and weepy and I think very frightened. She was very amenable to all suggestions I made".

MI5's Lt Col Stafford arrived, to find what he reported, "a very highly strung woman and inclined to become more unbalanced".

"After a display of tears," he said [which another recorded as "weeping incessantly for many hours"], Mrs Garbo wrote an emotional statement of apology, pledging "very seriously never to perform any action which might compromise my husband's work … the work which we have carried out with so much loving care up to the present".

But MI5 – and Garbo – weren't done.

Mrs Garbo was told a car would fetch her at 4pm. She was taken to Kew Bridge, blindfolded, put in the back of a closed van, and taken to Camp 020, an interrogation centre for captured German agents in South London run by Lt Col Robin "Tin Eye" Stephens.

There she saw her husband for 20 minutes. He was in prison clothes, unshaven.

She swore that she "would help him in every way to continue with his work with even greater zeal … she would never ask again to go back to Spain".

But Garbo's plan to break his wife's will was still not done.

The next day she was taken to MI5 interrogator Major Edward Cussen in room 114 at the Hotel Victoria. He extracted a promise that she would behave in exchange for her husband's freedom, which he told her was in his power to grant.

Cussen wrote later to a senior MI5 officer: "I said she must remember two things – that I did not like being proved wrong and that if she was bad in future I should be very angry indeed. That I had no time to waste with tiresome people and that if her name was ever mentioned to me again, I should simply direct that she should be locked up."

And finally, the brutal charade was over.

A few days later, the MI5's controller of double agents wrote a brief letter to "Tin Eye" at Camp 020, thanking him for the "magnificent stage management" which "got us out of an extremely difficult position".

He attached "for your amusement" a letter written by Garbo, supposedly pleading for mercy, but actually intended for his wife's eyes to round out the deception.

"To my mind it is a masterly piece of prose," the controller wrote.

From then on, MI5 were more attentive to Mrs Garbo's mental health – or to put it in their words, they "put up with her difficult nature".

They sent an agent on a mission to Spain to buy her favourite make of stockings.

They reckoned Mrs Garbo had been jealous of her husband – of his "quixotic zeal for his task", so they arranged for another agent's wife to try to befriend her and take "an occasional trip to the country with 'Mrs Barton' to collect information", on the pretense she was helping her husband's work.

It was not out of sympathy, but cold-hearted practicality. This was about beating the Nazis.

Wrote the handler: "If the recent experiment is found later to have had no lasting effect it has served its main purpose, in tiding us over a serious crisis, which was the main objective."