ON a freezing winter day last year, four workmen were admitted to a foreign embassy in central London. It belonged to a country described by the Foreign Office as a friendly power and a close ally. In a secure room on the top floor, past a thick metal door, the visitors found what they had come for: a large cipher machine used by diplomats to send home confidential messages without prying by British spies.

However, the workmen were themselves on an espionage mission for the British government. Their task was to give MI5 access to the embassy’s secrets.

It was intended as a crucial blow in the war on terrorism. Nothing could be allowed to go wrong. In the end, however, almost everything did. One of the “workmen”, a key intelligence asset, was on the brink of a nervous breakdown triggered by what he saw as the bungling and carelessness of his MI5 handlers.

The removal men criss-crossed the room carefully examining the machine, a hefty piece of equipment the size of a coffee table. Members of the embassy staff stood politely to one side. As far as they were concerned, the men were specialists who were deciding the easiest way to shift it to the basement.

The head of chancery, who stood holding the key to the door of the secure office, wondered why it took so many men such a long time. They assured him that moving the heavy machine to a secure bunker in the basement was no easy job.

What they did not tell him was that they had already copied down the secret codes for the machine from a yellow Post-it note on the wall. Now they were working out how, accidentally on purpose, to break the machine so they could send in one of their own repair teams to tamper with it.

After two hours the men left the white-stuccoed building and headed to a nearby luxury hotel. In the foyer they were greeted by a young woman in a business suit — the MI5 officer in charge of the operation. “It went brilliantly,” said one of the men. “Let’s go and celebrate.”

This was one of MI5’s most sensitive missions: a spying operation against an ostensibly friendly embassy to garner valuable information for the war against terror. The Sunday Times knows which embassy was involved but it is withholding this information for obvious security reasons.

The plan was to bug the building’s communications systems from roof to basement, from the radio transmitter to the telephone switchboard. MI5 also hoped surreptitiously to obtain the embassy’s archives of visa applications, a potentially valuable database that could be cross-referenced with other intelligence to help to identify possible terrorists.

Spying on diplomatic missions in London is part of MI5’s job. Officially sanctioned as “counter-espionage”, this usually involves keeping the staff of the diplomatic mission under surveillance and, during the cold war, occasionally expelling them for activities not consistent with their diplomatic duties (“PNGed”, from persona non grata, as such expulsions are known in officialese).

Sometimes, but not often, a rare opportunity arises that allows MI5 to get inside an embassy building and plant transmitting devices. In his book Spycatcher, the former MI5 officer Peter Wright claimed that British agents planted a bug inside the cipher room of the Egyptian embassy in London, enabling MI5 to read secret Egyptian messages throughout the Suez crisis, even though the codes were changed on a daily basis.

On this cold day last year, MI5 was attempting to do precisely the same again.

The entree to this embassy lay through a west London building firm that had answered an invitation to tender for restoration work at the building. The embassy enjoyed a prime site in central London, close to fashionable department stores and parks. But it was in an almost ruinous state. Paint peeled from some of the walls. Old visa applications were packed into the basement. The telephone system was out of date and the electrical system was old.

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