The Conspiracy: How a group of suicide bombers planned to blow up 10 planes

About 9pm on Wednesday, Britain's police and intelligence chiefs learnt that a British-based al-Qa'ida cell was within 48 hours of mounting a terror attack that could have been more deadly than 9/11.

Within hours, 24 suspected terrorists, mostly British-born, were arrested after raids across the South-east and Birmingham.

The emergency response was the culmination of more than a year of intensive surveillance and investigation by MI5 and the police in the largest counter-terror operation yet undertaken.

Security chiefs believe they have foiled a plot to smuggle home-made explosives on to up to 10 passenger jets bound for the United States, to be detonated by suicide bombers.

The operation began with MI5 officers watching a group of people in London with suspected sympathies for the aims of al-Qa'ida. The targets were among a group of about 1,000 terror suspects that the expanded Security Service had under investigation. At first, it appears to have been fairly routine, but gradually the alarm bells started to ring.

The group had links throughout the country - east London, High Wycombe in Buckinghamshire, and Birmingham - as well as abroad. They were said to be taking a disturbing interest in aircraft and homemade explosives, and how to smuggle the latter through airport security.

The handful of suspects grew to more than 20 - aged between 17 and 35 - and the surveillance operation began sucking in ever more MI5 agents and bugging teams. Whitehall sources say most of the suspects are of Pakistani descent and include women. But a few of the alleged plotters are also of north African descent. One suspect was a white British man in his 20s living in High Wycombe, who converted to Islam about six months ago. Sources said most of those arrested were in their late teens or early 20s.

In December, the Metro-politan Police's anti-terror branch was alerted. The targets were followed, their meetings and conversations recorded, their backgrounds investigated, and their bank accounts scrutinised. MI6 became involved and contacted a dozen counter-intelligence agencies in the United States, Pakistan, north African countries and Germany, for information and checks on the suspects.

Information from phone taps and bugging devices began to build a picture of a well-organised and motivated group, who were inspired by the idea of a global jihad and sympathetic towards the aims of al-Qa'ida. Clear links with Pakistan and north Africa were established, including several visits to Pakistany, but the intelligence failed to uncover a "Mr Big" from al-Qa'ida pulling the strings. It initially seems that, as with the July 7 suicide bombers, who killed 52 people in London last year, the plotters were a mixture of young people radicalised while living in Britain and influenced by travelling abroad.

The details of the plot also began to emerge. An experienced counter-terrorism officer described the findings as "bloody scary stuff". The plotters were allegedly planning to commit phased attacks in which three or four aircraft would be blown up over the sea, thereby destroying any clues about how the bombs were smuggled on board.

There were claims from American sources that the terrorists could have been planning to bring down airliners over major cities, either in the UK or US, but that suggestion was discounted by a British anti-terrorist source. After the initial attacks, the terrorists would wait until fear and panic had spread, then commit two more series of attacks, each involving three or four planes. In total, they intended to bring down nine or 10 aircraft.

Details of the make-up of the bombs are still sketchy, but it appears the plotters will be accused of copying many of the tactics used in a liquid explosive device developed 10 years ago.

This plot, which was foiled, is known as the "Bojinka" attack. In 1995, an Islamist terrorist aimed simultaneously to destroy 12 airliners over the Pacific. In this case, the mastermind was named as Ramzi Yousef, who developed liquid nitroglycerin explosives which could be hidden in contact-lens solution bottles. He also converted a digital watch into a timing switch and used two batteries hidden in his shoe to power light-bulb filaments and spark an explosion.

The British cell will be accused of using a liquid explosive which they were going to hide in a sports drinks bottles and smuggle through the airport security checks in hand mluggage.

An unconfirmed report from the US said the suspects planned to use a false bottom in the bottle and fill it with liquid explosive that had been dyed to match the colour of the drink.

The battery detonators would be sourced in electronic devices such as MP3 players or laptops

Based on details recorded from the suspects, the police had explosives specialists build a model which could be put together in minutes and blow a devastating hole in an aircraft.

The suspects were also allegedly heard discussing targets. On information presumably passed on by British police, US authorities said they had identified six to 10 airlines including United, American and Continental, all US carriers. Ten of their aircraft could hold as many as 2,800 passengers and crew.

As the months of surveillance continued, police and politicians issued increasingly gloomy warnings. The investigation was gathering pace and counter-terrorism chiefs decided to act soon as they thought the plot was about to be implemented.

On Wednesday night, the security services believed there was a strong possibility that the cell was preparing to execute its plan within the next 48 hours, or flee to reassemble later.

On Tuesday, the Home Secretary, John Reid, warned that Britain was facing its most sustained period of serious threat since the end of the Second World War.

Liquid explosives

Many commercial explosives come as two ingredients - oxidant and fuel - which when mixed produce an explosive that can be detonated by an electrical charge.

A bomb can be made by mixing relatively innocuous liquids such as acetone and hydrogen peroxide. Another well-known liquid explosive is nitroglycerine, right, which can be detonated with an electrical charge from the battery of an iPod or laptop computer.

"Most liquid explosives are unstable and dangerous to handle," said Professor Peter Zimmerman, chair of science and security at King's College London. "The best-known example is nitroglycerine. But there are extremely safe commercial explosives which come in two liquids which are mixed just before they are to be detonated. Explosive detonators are small, simple and, if just lightly camouflaged, could escape detection by X-ray scanners."

Clifford Jones, of the University of Aberdeen, said that if an explosion on an aircraft raised atmospheric cabin pressure by just 1 per cent, windows were likely to shatter.

If a blast raised pressure by 10 per cent, structural damage and possibly death would result. Dr Jones added: "A home-made bomb could be constructed with ammonium nitrate as the oxidant and something as innocuous as sawdust as the fuel. Overpressures can also arise when a liquid fuel and a liquid oxidant are so reacted, perhaps using hydrogen peroxide and acetone."

Steve Connor, Science Editor