More British Airways flights, this time to the United States and Saudi Arabia, have been cancelled for security reasons.
Security alerts are now a fact of life
BBC correspondent Frank Gardner looks at the state of airport security.
Q. What do we know about the threat to air safety?
The problem is that the security and intelligence services are playing this very close to their chest, as they do not want to give away to the terrorists how much they know and how they have intercepted the information they have got.
I think the safest way to look at this is to go with the known facts; there is a perceived al-Qaeda threat to western aviation and the continental US remains the main target for groups linked to al-Qaeda.
There are two different threats here - the threat to transatlantic aviation is believed to be from people trying to hijack planes or smuggle explosives onto them.
The threat to BA flights to Saudi Arabia is ground-based.
In other words people might have a surface-to-air missile in Saudi Arabia to try to bring down a British plane. Having said that, BA say services are now back to normal.
Q. Have BA confirmed the story that they are angry over recent proposals for air marshals?
They haven't said that publicly. There is a lot of disquiet among captains because no airline crew like the idea of having somebody firing guns inside a very soft-skinned aircraft.
The airline industry wants all the security to be on the ground.
They don't want anyone getting on board the aircraft that is going to be a problem. They say that is the job for people on the ground, but the Americans are really hanging tough on this.
They are on yellow alert now, with heightened airline security still in place, and they don't want any planes coming into the continental US that could crash into say Washington or Los Angeles.
So really the impetus for this is coming from the Americans and from the Department of Homeland Security over there.
Q. Are we going to have to live with this threat for some time?
Yes, I'm afraid we are. The best way to look at this is rather like those concrete blocks that were put up around the Houses of Parliament in Westminster.
When they were put up in June last year there was an awful lot of fuss about it, and I think I said at the time, this is something we're going to have to live with, unfortunately.
It's very sad and check-in times are going to get longer. The worrying thing is the way that al-Qaeda, and people linked to them, work.
When there is a lot of fuss, a lot of publicity, they tend to back off and cancel or postpone plans often for several months, even a year.
And then when everything has calmed down, that is when they tend to strike.
So the problem for Britain and the US - which are the main targets for al-Qaeda - is that they are going to have to continue to make this country a hard target to hit for a very long time.
Q. How much influence does the US have over security procedures at UK airports?
The US is currently at alert state yellow, its third highest level, which means the Americans are still jittery about any potential terrorists getting into their country.
As part of their overall precautions the US can insist on extra measures being taken to safeguard flights coming in to the continental US or, ultimately, refuse landing permission. (They are taking similar measures with container shipping).
Such requests are made via the British Government though, and not direct with the airline.
Q. Is there any truth in newspaper reports that particular individuals were being sought in the recent security scares?
Security and intelligence officials are refusing to confirm or deny this.
But given the specific nature of the threat to flight BA223 it is highly likely that individual suspects are being sought.
The Director General of MI5, Eliza Manningham-Buller, said recently that groups linked to al-Qaeda were active in Britain.
Since al-Qaeda has a long-standing fascination with using aircraft as weapons, it is highly probable that a plot exists to blow up an aircraft over a western city.