The “patio gas” bomb defused in Haymarket would have generated a fireball the size of a house and a shock wave spreading out over a diameter of at least 400 yards, explosives experts said today.

The propane cylinders and petrol used in the device would have triggered a huge conflagration, as well as causing shrapnel and blast injuries from the exploding car chassis and the nails packed around the bomb, according to Hans Michels, Professor of Safety Engineering at Imperial College, London.

Just one 13kg propane canister — the type sold by Calor under the brand name “Patio Gas” — would release a highly flammable cloud of vapour that would spread over an area of 50 to 60 cubic metres before igniting into a still larger fireball, he said.

“The vapour cloud from one cylinder would fill the order of a big room, and when it ignited the effect would be even bigger,” Professor Michels said. “In addition to the power of the explosion and the shrapnel, you would get a fireball the size of a small house.”

As several propane or butane cylinders were recovered, the volume of the fireball would have been greater still, though it is impossible to calculate the size without knowing how much gas would have been involved.

Professor Michels, who has been an expert witness in explosives trials, said that although the police did not say whether high explosives were found in the car, another charge would probably have been used to ignite the petrol and gas.

An initiator such as triacetone triperoxide or TATP could have been used to detonate a main charge, such as the flour and hydrogen peroxide mixture allegedly used in the failed attacks of July 21, 2005. This would have blown up the car, scattering nails and shrapnel, as well as igniting the petrol and puncturing the gas cylinders.

The gas-fuelled fireball would have followed, though the timing of ignition would have affected the ultimate extent of the blast. It is possible that the petrol was intended as the main charge, but if so this would have resulted in a smaller explosion than if high explosive had been used.

The original explosion would have had to be large to penetrate the propane canisters, which are designed to withstand high-speed traffic accidents and fires.

Professor Michels said: “It is almost certain that the explosive device itself would have been sufficiently powerful not to just fragment the gas cylinders, but to destroy the car and possibly the front of buildings, with missiles, shrapnel, nails and burning petrol flung at very high velocity in the wake of the shock wave and the whole surrounded by a massive fireball resulting from the instantly evaporated and exploded propane and/or butane.

“It is also likely that the source of ignition and the explosive that should have set off the device was of the home made type, consisting of household materials now most commonly used by terrorists.”

Other experts suggested that the total blast could have been bigger still, depending on how many propane cylinders ignited and on whether high explosive was also used.

Andy Oppenheimer, editor of Nuclear, Biological and Chemical International, said the blast radius could have been anything from 200 years to half a mile. “It would have been a devastating explosion,” he said.

“With that amount of petrol and an unknown quantity of pressurised gas, the blast would have been about 200 yards. If high explosive was involved, the blast could have reached half a mile.

“Hundreds of people could have been injured if they had been in the area at the time. The knock-on effects of breaking glass are particularly devastating, for example.

“This would have been an explosion on the scale of those seen in the Middle East, although not as big as some that have been seen in Baghdad recently.”

The recovery of the intact bomb will also help forensic scientists to trace the bomb-makers, Professor Michels said. Propane cylinders carry a serial number which can be used to find the point of sale, and isotopic analysis could be used to trace any flour used in the main explosive charge.

Propane cylinders contain liquified propane, a volatile hydrocarbon, which would rapidly be transformed into a gas occupying 200 to 400 times the volume when released. This would mix with between 15 and 20 times that volume of air to produce an inflammable vapour cloud.

Brian Baker, director of the Association For Petroleum And Explosives Administration, said: “Propane is liquefied petroleum gas and patio heater gas is usually 97 per cent propane. Cylinders of this type of gas are readily available to the public and can be bought in places such as petrol stations and iron monger in particular.

“Propane is heavier than air when released, highly flammable and easy to ignite. When released in to the atmosphere and only a small amount is a required to cause an explosive condition and this is worse in a confined space when subject to an ignition source. Its explosive properties mean that 10 litres of the gas is equivalent to 2770 litres of flammable gas and air mix. There are controls on using this type of gas and this is why the industry gives lots of safety advice about use.”

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