BBC News website readers worldwide have been describing how their travel plans have been severely affected by the high security alert at UK airports.
IAN TREDGETT, EN ROUTE FROM BERLIN TO LONDON - VIA AMSTERDAM AND BRUSSELS, FRIDAY
It is taking me about a day and a half to make a journey that usually takes about two hours.
Many passengers were forced to sleep in airports on Thursday night
I travel on business every week and this is the worst set of delays I've seen.
I was hoping to fly home on Thursday at 1455 with Air Berlin but all flights to the UK were cancelled.
So the only flight I could get was to Amsterdam, where I had to stay overnight.
I am now on a train from Amsterdam to Brussels and I hope to get the Eurostar from there to London.
It's proving to be a very long journey but I have to get home any way I can.
I have seen huge queues and crowds of people that were forced to sleep at two major European airports - Berlin and Amsterdam - after being told there were no flights available.
It's all been very messed up, but it is, after all, in the interest of security. I guess this is just something we will have to get used to now.
DAVE AINSWORTH, ISTANBUL, TURKEY, THURSDAY
I have been waiting all day at Istanbul airport trying to get a flight to Heathrow.
Passengers are facing long delays
It's complete chaos. No one knows what is going on and I keep getting passed from one person to another, only to get repeatedly told to wait for another hour or so.
I have no idea whether to book accommodation or wait for a possible later flight.
Five out of the six flights due to fly to Heathrow today have so far been cancelled and all of Friday's flights are fully booked.
I may have to see if I can fly to Paris instead and get the Eurostar train back to London.
It's obviously not the best situation.
EMMA ALLEN, MADRID, SPAIN, THURSDAY
I am a flight attendant for a UK-based transatlantic carrier.
I tried to fly to London this morning but was told by Iberia Airlines that the flight was cancelled and that no further flights would be flying to Heathrow.
They didn't tell us why but my dad phoned me to tell me about the security alert in the UK.
I have just given up and gone home now.
The crowds were not that big at Madrid airport but they will be huge on Friday, as it is a holiday, and there is bound to be a knock-on effect on flights here.
As a flight attendant I'm happy to see such high security measures in place, but the people who are looking to attack aircraft will look for any weaknesses and unfortunately you have to say that if it's going to happen it'll happen.
It won't make me any more concerned to fly, however.
Today is probably the safest day to travel.
SHAUKAT BADANI, SAN DIEGO, CALIFORNIA, USA, THURSDAY
My wife and son are travelling from Manchester, UK to San Diego, via Philadelphia.
All luggage has been subjected to stringent security checks
They had a four hour delay and will miss their connecting flight from Philadelphia to San Diego.
I am trying to get them on another flight or get a hotel room for them in Philadelphia.
This will not be easy as Philadelphia is a major hub and there are bound to be a lot of people having problems getting where they want to go.
As a traveller, you don't mind this kind of inconvenience if it's for safety, but if the authorities were working on this for a long period of time, as they have said, it makes you think that they should have had a more organised plan in place for passengers.
It all looks pretty chaotic as far as I can see.
LAURA MCLEOD, PHILADELPHIA, USA, THURSDAY
I'm due to fly back to London Gatwick today from Philadelphia.
I have been trying to get through to the airline, US Airways, but there is no information about my flight at the moment, so I'm a bit stuck on what I'm going to do.
I have had family members phoning to check that I'll be able to get back safely, and I'm really quite nervous about flying now.
It's quite scary.
I was just in New York yesterday at Ground Zero paying my respects to the people who died there and then I wake up to this news today.
It's all very sad and worrying.