James McArdle, 29, was part of an IRA team responsible for detonating a massive lorry bomb in February 1996 that caused an estimated pounds 150m of damage, a court was told.
McArdle, a bricklayer and farm labourer from Crossmaglen, Co Armagh, showed no reaction as the jury at London's Woolwich Crown Court convicted him of one charge of conspiring to cause explosions between 30 October 1995 and 10 February 1996.
The jury was sent home last night to continue deliberating on two further charges of murder against McArdle. He has denied the charges. The judge told the jury he would except a majority verdict.
McArdle is accused of murdering Inam Bashir, 34, and John Jeffries, 31, the two newsagents who died in the blast.
John Bevan QC, for the prosecution, said that it was "little short of a miracle" that more people were not killed or injured in the bombing because of "inaccurate and wholly inadequate" warnings given by the IRA.
McArdle was finally brought to trial following a meticulous investigation by anti-terrorist officers aided by a sharp-eyed member of the public. The two-and-a-half week trial heard the Crown insist McArdle played a "crucial" role in the terrorist attack.
The huge lorry bomb McArdle is said to have parked at South Quay close to Canary Wharf in east London brought an end to the 18-month-old IRA ceasefire that had begun in August 1994 and placed the peace process in severe jeopardy.
The court heard that McArdle twice drove the converted car transporter used in the attack from Belfast to the mainland. On the first "dummy run" the bomber and another man, who remains unidentified, drove the vehicle to Carlisle, in Cumbria, to buy two cars at an auction to establish the credentials of the lorry.
Three weeks later McArdle was behind the wheel once more, but this time the vehicle carried more than a ton of home-made explosive in a secret compartment.
The court was told that over the next two days the pair, almost certainly accompanied by a back-up team travelling separately, made their way to London.
Once there they stopped off at waste ground in River Road, Barking, Essex, where the trailer they were pulling was abandoned and incriminating evidence dumped in a discarded tyre.
The jury was told McArdle and the other man then drove the lorry the nine miles to their target. Once there the timer was set and the men fled. In the following explosion, which left a 15ft by 30ft crater, the bodies of the newsagents who died were hurled through walls into the next-door- but-one shop.
A police hotline following the blast received 850 calls. The 199th call came from Arthur Ward, a lorry driver, who had spotted the bomb lorry parked in River Road and tipped off the police.
An intensive police investigation allowed detectives to piece together the history of the vehicle used in the attack and recreate much of the bombing trip.
Crucially they managed to find three of McArdle's thumb prints.
McArdle admitted driving the lorry but claimed he was just an innocent dupe who had been used by an IRA Godfather he referred to only as the "boss". He refused to identify the man in court saying it would put his family at risk.
The jury will resume their deliberations today.