As the evening drew on he picked up the telephone to make a critical call to the head of MI5, Dame Eliza Manningham-Buller. For months the two had been co- ordinating Operation Overt, a huge surveillance and intelligence effort aimed at two cells of suspected Islamic militants who were believed to be plotting a massive terrorist attack.

They had been patiently gathering evidence and following all the tentacles and were preparing to move in only when they were satisfied that they had every suspect covered. Now a sudden twist of fate threatened to disrupt their plans. It was time for a decision: to pounce or to continue stalking.

Thousands of miles away a British man called Rashid Rauf had been arrested in Pakistan. Reports of what had happened were conflicting.

One senior Pakistani government official said the Pakistani police had detained Rauf, a Muslim, in connection with an investigation into the murder of his uncle, who had been stabbed in Britain in April 2002. He told The Sunday Times that Rauf had been arrested in the southeastern city of Hyderabad while going to catch a bus to Karachi.

But a Pakistani intelligence source said Rauf had been under surveillance for several weeks following a tip-off from Britain. He had been caught in Bahawalpur, said the source, in the home of a militant linked to the proscribed Lashkar-e-Jhangvi group, a band of Sunni extremists.

Wherever and why he was arrested, two things were clear. British police were caught by surprise, and Rauf was much more than a man wanted in connection with a murder inquiry. He is, the intelligence agencies believe, linked to Al-Qaeda. “He’s the main guy in Pakistan,” said the intelligence source. “He has a very large network extending beyond Europe.”

And he is the brother of a man in Britain who the British security services suspected of being involved in the biggest terrorist plot to be planned in the UK.

News of the arrest came as a shock in London. The soft- spoken Clarke conferred with Manningham-Buller. How long would it be before Rauf’s arrest in Pakistan became common knowledge in Britain? What would happen then?

At Scotland Yard dozens of senior officers and plainclothes detectives assembled, waiting for Clarke’s orders.

By mid-evening Clarke and Manningham-Buller had reached their decision. They could wait no longer. They could not risk the entire operation being blown.

“It was all hands to the pump,” said a senior police source familiar with the operation. “It was a pretty exceptional piece of business. They just gathered up all the people who were available to do this at short notice.”

Teams of detectives and anti-terrorist officers headed out into the night. Shortly before 11pm John Weir, of Forest Road, Walthamstow, east London, noticed an unusual number of cars parking on the street, their occupants staying put.

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