Homemade devices made by the alleged July 21 bombers contained “a very effective high explosive” that could have caused a repeat of the carnage of July 7, a court was told yesterday.

The power of the improvised devices was displayed to jurors in the July 21 trial when they were shown film of tests by government scientists in a deserted quarry. Screens at Woolwich Crown Court displayed slow-motion footage of replica devices exploding.

Four such “bombs” partially detonated on three London Underground trains and a bus on July 21, 2005 — two weeks after 52 people were killed in the July 7 suicide bombings. Nigel Sweeney, QC, for the prosecution, asked Clifford Todd, the forensic scientist who supervised the testing, what the consequences would have been had the devices fully exploded on July 21.

“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.” The trial has previously been told that only “good fortune” stopped the devices exploding.

Devices containing more than 6kg (13lb) of hydrogen peroxide-based explosive material had been put in the centre of the quarry. Blast gauges were near by to measure the force of the explosion and “witness plates” — sheets of aluminium and plywood — were set up to judge the extent of damage and spread of any chemical residue.

Scientists and legal observers in a fortified bunker 400 yards away watched the tests via cameras in concrete shelters.

On screen the detonation of the device was marked by a white flash that gave way to the yellow-orange colour of flame at the centre of the explosion. Two clouds of dust and smoke spread outwards from the blast, one moving sideways, the other shooting up into the air.

Mr Todd, principal forensic investigator at the Forensic Explosives Laboratory, explained: “You can see a shimmer across the screen. That is the shock wave generated by the detonation.” As the wave reached the witness plates they were blown over or shattered. Mr Todd, asked if he could hear and feel it in the bunker, replied: “Yes, absolutely, you do hear it and you can feel the shock wave.”

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, he said, that forensic laboratories in Britain had encountered hydrogen peroxide explosives. Extensive research was done to replicate the recipe allegedly used by the defendants.

There were tests on mixtures that involved different ratios of hydrogen peroxide to chapati flour — the additional fuel used on July 21 — and with various concentrations of the chemical.

The composition most closely resembling that which the alleged bombers tried to make exploded every time it was tested with a detonator, he said. “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 further tests were ordered after receipt of a new defence-case statement which claimed that the TATP (triacetone triperoxide) detonators in the devices were designed not to detonate the main charge. The new tests had to be done under stringent safety conditions because of the sensitivity and instability of TATP. Mr Todd said: “This has not been done anywhere else and I have had discussions with colleagues in other countries who are very interested.” A mechanical arm was used to insert the detonator into the main charge for these tests. Mr Todd said: “If something goes wrong at that point, that will kill you.”

Mr Todd said that he did not believe the claim of Muktah Said Ibrahim, who admits making the July 21 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 that which would have failed to explode.

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

The trial continues.

Times Recommends

• Third of Muslim students back killings
• Curfew tames feral yobs of Cornwall
• £3,000-a-month drug free abroad