The security experts had reason to feel pleased with their efforts. The arrest of an Al-Qaeda suspect who was a computer expert had provided vital information about the terrorists’ internet communications. He had revealed how they had devised a method for skirting the intercepts of eavesdropping agencies such as GCHQ.

Al-Qaeda operatives, the suspect said, were writing messages in Hotmail or Yahoo! e-mail accounts — but not sending them. Instead they were leaving them stored in the “draft” folders of the accounts. Unsent, they could not be intercepted.

However, any Al-Qaeda activist who knew the account and password could log on from anywhere and read the messages.

Another Al-Qaeda informant had also supplied MI5 with the means to decipher a code that Al-Qaeda was using in some messages. The breakthrough was described by one insider as similar to Enigma, the brilliant codebreaking operation run by British intelligence during the second world war.

Despite these successes, however, they had no indication of imminent attack.

It was a tragic mistake. After four terrorist bombs ripped through central London on Thursday, killing at least 49 (and perhaps up to 70) and wounding more than 700 others, rescue workers were yesterday still grappling with the full horror of the worst attack, deep underground on the Piccadilly line near King’s Cross.

They faced an infernal scene. The wrecked front carriage blocks the tunnel. The air is choked with dust and soot. The heat is stifling. More than three days after the bombing, many bodies and other remains are still inside the carriage. Rats and other vermin infest the tunnel.

“It’s very hot, very dusty,” said Andy Trotter, deputy assistant commissioner of British Transport Police. “It’s a great challenge to recover the bodies.”

Squeezed into the wreckage, clad in protective suits, some emergency workers are trying to extricate the human remains, while others are forensic experts seeking clues to the bombs. “It will be some time before this is completed,” said Trotter. “But it will be done with dignity.”

This weekend the initial wave of relief that the attacks had not been worse was being overtaken by the grimness of the recovery operation. The death toll in the worst terrorist attack to have taken place on mainland Britain is likely to rise. More than 20 are still missing. More than 20 are critically ill in hospital. And so far police have no firm idea who perpetrated the atrocity, where they might be or whether they might strike again.

SUMMER had come early to London, lighting the city with optimism. A heatwave in early June decked the crowds in pretty dresses and open-necked shirts. Cafes moved tables outside for al fresco dining.

The stock market was on the up and the shops were full of bargains. The daily horrors of Baghdad and the memories of September 11 and Madrid seemed a long way away. Indeed, unnoticed by most people the government had quietly downgraded the level of alert against terrorist attack.

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