The operations, which resulted in the death of one suspect, the capture of five, and the seizure of up to 10 tonnes of explosives, bomb-making equipment, booby-trap car devices and weapons, is the second big seizure in just over two months.
In July, officers recovered components for up to 36 bombs which, it is thought, were to have been used to target power stations in the south- east. Eight men were later remanded in custody charged with conspiring to cause explosions.
So are the authorities getting lucky or more skilled at tracking down terrorists? A significant development this year was the appointment of Commander John Grieve, head of Scotland Yard's Anti-Terrorist Branch. He had just taken up his new post at the beginning of February when a lorry bomb destroyed the ceasefire when it devastated a large area of Docklands, killing two people.
Commander Grieve, a philosophy and psychology graduate, has a reputation as a shrewd but unconventional policeman. In his previous job as the Metropolitan Police's director of intelligence, he headed a team of specialists in surveillance and data analysis in targeting drugs-traffickers, international criminals and London gangs.
He has used his expertise in intelligence-gathering to bring a new, and many believe more thoughtful and astute, approach to anti-terrorism. Among his new developments is emphasis on using closed-circuit television to ensnare terrorists and numerous appeals to the public and the criminal underworld for help. This has been backed with promises of up to pounds 1m in reward money.
The potential benefit of "inside information" was highlighted by reports last night that an informer was suspected of helping police thwart the IRA in yesterday's raids. Commander Grieve's cerebral approach has also met increasing approval and appreciation from MI5, which has overall responsibility in anti-terrorist intelligence-gathering and analysis.
Insiders say the net result is greater "harmony" between the police and the security service. Gone are the bad old days of turf wars and there now appears to be much closer co-operation and greater understanding.
MI5 intelligence-gathering expertise has also been praised as a factor in the operations. Surveillance and undercover techniques used by the security service have been evolving over the years and are "paying dividends", a source said.
But there are still a number of unsolved terror attacks from this year. Most significantly, the Docklands and Manchester bombers are still at large and while yesterday's arrests and seizures are seen as an important breakthrough, they are only considered a dent in the IRA war machine.
It is understood that the suspect who died after being shot yesterday may have been a "sleeper", a man of Irish extraction who had lived in Britain for several years, blending into the community over a long period to avoid arousing suspicion.
The Police Complaints Authority will supervise an investigation into his death.
Four other men arrested by armed officers in a house in Fulham, south- west London, and one detained in Crawley, East Sussex - later identified as an engineer working at Gatwick - are understood to have lived in Britain for a number of years, but come from Ireland.
David Veness, Assistant Commissioner of the Metropolitan Police said yesterday that the home-made explosives were virtually "ready for use" and would probably have been used as large vehicle-bombs, such as the one which injured 200 people in Manchester in June and the one which killed two people in Docklands in February.
Up to 10 police forces are thought to have been involved in tracking suspects during a surveillance operation lasting about three weeks. A house and an industrial unit in Sheffield were also searched yesterday but nothing was found.
Gerry Adams, the president of Sinn Fein, said yesterday that he regretted somebody had been killed in the police raids and added that he presumed the IRA had been involved. He added: "It should be an incentive to political leaders, particularly the governments to build a real peace process."
Gatwick engineer held
A newly qualified engineer at Gatwick Airport was one of those arrested yesterday. Hewas detained as he ended a night-shift.
When, as an 18-year-old, he applied for an aircraft- engineering apprenticeship, he was interviewed by senior British Airways engineers and managers, and his headmaster and two other referees were interviewed by police, according to reports last night.
He got a BA pass as soon as he had obtained security clearance.
It was then submitted to the British Airports Authority, the airport operator at Gatwick, for approval and to get an "airside" pass.
A second round of checks and interviews with at least one of the referees was undertaken and that pass was promptly obtained.
The engineer rapidly moved to the top of his class and graduated this summer with some of the best marks of the year.
He holidayed in Ireland, returned to Gatwick and received his first pay cheque this month.
"If in the light of police investigations there is a need to review our requirements, then we will do so," said a spokeswoman for the Department of Transport, which is responsible for the guidelines that ensure anyone who works near aircraft gets a strict security check.
Timetable of error
February 10 1996 - South Quay bomb rocked Docklands, east London, and ended 18 months of IRA ceasefire. Two men are killed and more than 100 injured.
February 18 1996 - IRA terrorist Ed O' Brien is killed when a home-made bomb exploded on a double-decker bus in Aldwych, west London.
March 9 1996 - IRA explodes a bomb in a litter bin in Old Brompton Road, Fulham, West London. No one is injured.
April 17 1996 - A small bomb was set off by the IRA in the front garden of a house under renovation in The Boltons, an expensive residential area near Earl's Court, west London.
June 16 1996 - More than 200 people injured, 10 seriously, by a massive IRA car bomb in Manchester City Centre. The attack, at the height of Saturday -orning shopping, caused an estimated pounds 150m damage.
June 28 1996 - British soldiers at the Quebec Barracks in Osnabruck, Germany, hit by mortars fired by the IRA from the back of Ford Transit van. The attack marked a return to terrorist violence against British soldiers after a six-year break.
February 2 1996 - IRA blamed for an attack on the home of an off-duty police officer in which 57 shots are fired. The part-time RUC reservist and his wife were in bed when gunmen attacked his him in Moy, Co Tyrone.