The homemade devices made by the alleged 21/7 bombers contained “a very effective high explosive” which could have caused a repeat of the carnage of 7/7, a court heard today.

The power of the improvised explosive devices was displayed to the jurors in the 21/7 trial when they were shown a film of tests carried out by government scientists in a deserted quarry.

Television screens around the courtroom at Woolwich Crown Court played footage of replica devices exploding in flashes of light, flames and smoke.

Four such “bombs” partially detonated on three Tube trains and a bus on July 21 2005 - two weeks after the deaths of 52 people in the July 7 suicide bombings.

Nigel Sweeney, QC, prosecuting, asked Clifford Todd, the forensic scientist who supervised the testing, what the consequences would have been if the devices had fully exploded on 21/7.

“Very similar to what they were two weeks previously,” replied Mr Todd. “Very large scale damage to the train and many deaths and serious injuries if there were people there at the time.”

The trial has previously heard that it was only “good fortune” that stopped the devices exploding.

The slow-motion footage of the tests was played as the trial entered its ninth week.

Devices, containing more than 6kg of hydrogen peroxide-based explosive material, were placed in the centre of the quarry.

Blast gauges were stationed at intervals nearby to measure the force of the explosion and “witness plates” - sheets of aluminium and plywood - were erected to judge the extent of damage and spread of any chemical residue.

Scientists and legal observers retreated to a fortified bunker 400 yards away and watched the tests via cameras sited in concrete shelters on the quarry floor.

On screen the detonation of the device was marked by a bright white flash which gave way to the yellow-orange colour of flame at the centre of the explosion.

Two clouds of dust and smoke then spread outwards from the blast, one moving sideways the other shooting directly up into the air.

Mr Todd, principal forensic investigator at the Forensic Explosives Laboratory, explained: “You can see a shimmer move across the screen. That is the shock wave generated in the air by the detonation.”

As the shock wave reached the “witness plates” they were blown over or shattered.

Mr Todd was asked if he could hear and feel it in the bunker.

He replied: “Yes, absolutely, you do hear it and you can feel the shock wave.”

As he stood in the witness box Mr Todd had beside him a mock-up of the 21/7 devices - a six-litre plastic kitchen container filled with a yellowish substance from the bottom of which two electrical wires protruded.

Wrapped around the tub with layers of tape were nuts, screws and washers intended to act as shrapnel.

July 2005 was the first time, Mr Todd said, that forensic laboratories in Britain had encountered hydrogen peroxide explosives. Extensive research was carried out to replicate the explosive recipe allegedly used by the defendants.

Tests were carried out on mixtures that involved different ratios of hydrogen peroxide to chapati flour - the additional fuel used on 21/7 - and with different concentrations of the chemical.

Mr Todd said that the composition of chemicals most closely resembling that which the alleged bombers tried to make exploded every time it was tested with a detonator.

He added: “The ratio makes a very effective high explosive which is readily explodable using a detonator or booster... Potentially this device, as constructed, was viable.”

Last autumn a second series of tests was ordered after receipt of a new defence case statement which claimed that the TATP (triacetone triperoxide) detonators used in the devices were designed not to detonate the main charge.

Mr Todd said the new tests had to be carried under stringent safety conditions because of the sensitivity and instability of TATP.

Mr Todd said: “This is brand new, it has not been done anywhere else and I have had discussions with colleagues in other countries who are very interested in this series of tests.”

A mechanical arm was used to insert the detonator into the main charge for the second series of tests.

Mr Todd said: “If something goes wrong at that point, that will kill you.”

Mr Todd said he didn’t believe the claim by Muktar Ibrahim, who admits making the 21/7 devices, that they were an elaborate hoax.

He said there was a fine dividing line between the concentration of hydrogen peroxide that would have worked and the strength that would have failed to explode.

Mr Ibrahim, 29, Manfo Kwaku Asiedu, 33, Yassin Omar, 26, Hussein Osman, 28, Ramzi Mohammed, 25, and Adel Yahya, 24, deny charges of conspiracy to murder and cause explosions. The trial continues.

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