The Hyde Park and Regent's Park bombings were one of the worst IRA atrocities on the British mainland, killing 11 soldiers and seven horses and leaving dozens injured.
The bombs were detonated a couple of hours apart on July 20, 1982, and timed to cause maximum casualties.
John Downey was charged only with alleged involvement in the Hyde Park bomb and no one has ever been charged in connection with the Regent's Park blast.
The first attack came in Hyde Park about 10.40am just as members of the Royal Household Cavalry, Blues and Royals, walked past as part of the Changing of the Guard procession.
A huge nail bomb, containing up to 11kg of high explosive, was hidden in a blue Morris Marina car that had been parked on South Carriage Drive and was detonated by remote control as the soldiers rode past.
Lieutenant Anthony Daly, 23, Trooper Simon Tipper, 19, and Lance Corporal Jeffrey Young, 19, were killed outright and squadron quartermaster Corporal Roy Bright, 36, the standard bearer, died from his injuries four days later.
Daly and Tipper had each been married for less than a month when they died.
Seven horses were killed but another, Sefton, became a national icon and symbol of hope after recovering from 34 injuries and an eight-hour operation.
The black gelding was given a 50/50 chance of survival, but recovered sufficiently to return to military service for a further two years and lived until 1993. His rider, Michael Pedersen, who was injured, also became a national hero but tragically took his own life and those of his two young children in 2012.
About two hours after the first blast, a second device exploded under a bandstand in Regent's Park, killing seven bandsmen from the Royal Green Jackets, who were playing at the time. The bomb was thought to have been placed under the stand weeks in advance, with a timer set to the date and time of the advertised concert.
More than 50 people were injured in the two attacks and pictures of the horses' corpses lying among the debris became one of the enduring images of the Troubles.
The attacks came just over a year after IRA hunger striker Bobby Sands died in the Maze Prison.
In 1987, Gilbert "Danny" McNamee, an electronics engineer from Northern Ireland, was jailed for 25 years after being found guilty of building the radio-controlled bomb used in the Hyde Park attack.
He was released from prison in 1998 under the terms of the Good Friday Agreement, and later that year the Court of Appeal overturned his conviction.