Rules doubling the amount of passenger duty people pay when taking flights from the UK have come into force.
Airlines are not expecting major problems collecting the duties
The increase was announced by Chancellor Gordon Brown last December. He said airlines should pay more for damaging the environment.
Hundreds of thousands of people have been asked to pay extra duty, despite buying tickets before the announcement.
While the new tax is only payable on flights from UK airports, passengers face paying twice on internal flights.
'Lawful and proper'
The government has rejected Conservative accusations that the new tax is not legal because it has been introduced within eight weeks of being announced by the chancellor and has yet to be discussed by MPs.
Jack Straw, Leader of the House, told MPs: "The decision to bring the tax into force today was entirely lawful, proper and consistent with previous procedures."
WHO IS LEVYING THE EXTRA TAX?
It applies to all flights from the UK from 1 February
Almost all airlines are charging customers who bought their tickets before the tax rise was announced last December
BA is not charging those customers - at a cost of £11m
"I don't quite know why there are some complaints from the airlines because when they impose a fuel surcharge...they impose a fuel surcharge on passengers who have already booked, who have already paid the previous fare," he added.
At Luton airport this morning, Easyjet passengers flying to Faro in Portugal expressed some disgruntlement, but doubted that the higher tax would encourage them to fly less often.
"On short-haul flights it's not a lot of money but on long-haul flights it's becoming quite expensive," said one passenger.
Economy class flights in Europe, including internal UK flights: £10
Business and first class flights in Europe: £20
Economy class long-haul flights: £40
Business and first class long-haul flights: £80
Another said: "I don't believe it's justified - if I thought it was going to help to pay for the environment I would be happy to pay."
Airlines said they were not expecting major problems in collecting the higher rates of air passenger duty.
Despite some predictions of chaos, most airlines are - at least in public - expressing confidence that there will be few hold-ups.
"The vast majority of passengers have now paid," said an Easyjet spokeswoman.
The Treasury brought in the rises after accusing the aviation industry of not meeting its environmental costs.
BA absorbed the cost of the rise, at a cost of £11m
But the move has failed to impress environmental groups, airlines or passengers.
Mike Carrivick, chief executive of the airline trade body the Board of Airline Representatives, said the tax was purely a money-grabbing exercise.
"At the moment Parliament has not even discussed the issue, but it has been made very clear to us that when it does it (the tax) would be made retrospective," he said.
"That places the industry in a very invidious position - it lays us open to a liability of well over £100m."
Friends of the Earth was hoping for a bigger increase, while the British Air Transport Association said the rises would do nothing to tackle emissions.
British Airways has absorbed the £11m cost of the increase for those travellers who bought tickets before last December's announcement, for travel from 1 February onwards.
But other airlines are passing on the tax to their customers.
All airlines, whether UK-based or not, are obliged to collect the tax for each passenger they fly out of the country.
The big fear for the airlines was that some customers would turn up completely unaware they had to pay.
Many have been sending e-mails to their passengers, or have arranged to collect the fee from customers' credit cards.
But the airlines have also prepared for the possibility of collecting cash directly at the airport.
BMI said it had a leaflet which explained the situation.
"The ultimate sanction in our terms and conditions is to refuse to carry them," a BMI spokesman said.
That view - which would effectively mean no payment, no flight - is shared by all the airlines which have decided to pass on the tax.