The “patio gas” car bombs found yesterday could each 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.

The propane cylinders and petrol used in such devices can trigger a huge conflagration, as well as causing shrapnel and blast injuries from the exploding car chassis and the nails packed around the bombs, according to Hans Michels, Professor of Safety Engineering at Imperial College, London.

Just one 13kg (28lb) 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 cu m (1,766 to 2,119 cu ft) 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.”

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As several propane or butane cylinders were recovered from the Haymarket car, 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 had been found in the car, another charge would probably have been used to ignite the petrol and gas.

An initiator such as tri- acetone triperoxide (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 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 ig- nited and on whether high explosive was also used. Andy Oppenheimer, editor of Nuclear, Biological and Chemical International, said that the blast radius could have been anything from 200 yards to half a mile. “It would have been a devastating explosion,” he said.

The recovery of an intact bomb will 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.

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