Dozens of flights into Heathrow and Gatwick were diverted to other airports and thousands of passengers were delayed during the alerts, which covered about three hours. Scotland Yard called the scares a cynical attempt to frighten passengers and cause confusion.
Passengers at both airports were moved to secure areas after coded bomb warnings. Perimeter roads were closed and flights halted while security checks were made. There were reports that some passengers at Heathrow spent the entire alert in aircraft on the tarmac because of overcrowding in the terminal.
At Heathrow, the last flight in before the airport closed arrived at 7.33pm from Geneva. Terminal 4 was sealed off for the second time in one day. A stretch of the A4 next to the northern perimeter road was shut. A spokesman said: 'An agreed security plan was put into effect.'
At Gatwick, Alan Munds, the airport's managing director, said both terminals were closed and the runway shut at 7.15pm. Passengers leaving for cars or buses were ushered inside the main terminal buildings at about 7.45pm. The British Rail station at Gatwick was also closed.
Thirty-six flights bound for Heathrow and about 24 heading for Gatwick were diverted to airports as far afield as Stansted, Southampton, Luton, Manchester, Bournemouth, Bristol, Cardiff and Birmingham.
Heathrow reopened at 9.30pm and passengers were gradually allowed to continue their journeys. Departing travellers were advised to check with their airlines. The security alert at Gatwick ended with the airport being reopened at about 10pm after a thorough search by security services.
Two coded warnings were telephoned to Sky News - at about 7.10pm and 7.35pm. The first caller said: 'There will be bombs in one hour's time at Heathrow and Gatwick airports. Clear all runways and terminals of people.' The second caller repeated: 'Clear all runways.' Both gave a code word. A police spokesman at Gatwick said there had also been a warning through the media at 7pm.
The London airport alerts brought widespread chaos to domestic and international flights, which had not returned to normal after four mortar bombs had been fired yesterday morning at Heathrow, hitting the roof of a crowded terminal building and missing empty passenger jets by less than 60 metres.
The IRA last night put out a statement admitting the first two mortar attacks at Heathrow - last Wednesday afternoon and just after midnight on Friday morning - and accusing the police of ignoring bomb warnings and putting people's lives at risk.
In a statement issued in Dublin, it said: 'The luck of the British authorities who are deliberately taking these calculated and cynical gambles with people's lives will inevitably run out. Our coded warnings should be heeded.'
The IRA claimed the authorities failed to 'act appropriately' when given what it described as accurate advance warning of Wednesday's attack. The Provisionals accused the Metropolitan Police Commissioner, Paul Condon, and the head of the anti-terrorist branch, Commander David Tucker, of 'arrogant boasting' and claimed a six-hour warning of the attack at Terminal 4 had been totally ignored.
A few hours later, the IRA demanded new direct talks between Republicans and the Government, saying there had to be discussions if there was any chance of an end to the violence. It asserted that a continuing opportunity for peace existed, and should not be squandered.
The police turned to the Army for help yesterday morning after the latest Heathrow mortar attack. The embarrassment of senior anti-terrorism officers was palpable when it emerged that the set of bombs had been launched by a mortar dug just over a metre into ground already searched.
As a call went out to the Army for sophisticated metal detectors and thermal imaging equipment, Mr Tucker warned of more attacks.
Mr Condonsaid there was no short-term prospect of soldiers patrolling the airport. 'We are not facing audacious terrorists coming back night after night,' he said. 'They are cowards who have secreted more than one device and slunk away in the middle of the night.'
He thought the mortars had probably been planted early last week and were being launched by timing devices that would be looked for with the help of the military equipment. For the third time in five days, the mortar bombs failed to explode and no one was hurt. The attack was launched by timer shortly after 8am. Coded warnings went to three news organisations between 6.19am and 6.41am, leading to the closure of the southern runway.
Flights were about to resume when the bombs were launched from scrubland off Bedfont Road on Heathrow's south-western perimeter. Three landed on an apron where several jets were parked and the fourth hit the roof of Terminal 4 while passengers were inside. They were moved out and the terminal closed. Terminal 4 reopened at about 2.30pm, only to shut again hours later.
'The mortar had been very carefully camouflaged and dug in to a distance of 3ft-4ft,' said Mr Tucker. 'The extensive searching operation which took place within that area failed to find the baseplate. We are now not just looking for vehicles and mortar baseplates that are visible, but we have to consider the means by which we can detect something in the ground in an area 10 miles long and three-quarters of a mile wide.'
Asked why 12 Semtex-filled mortar bombs had so far failed to explode, Mr Tucker said: 'There appears to be a persistent mechanical defect.' He discounted the idea that the IRA had not intended they should explode, saying all had been 'viable'.
Frustration at delays, page 2
Leading article, page 13