Air Passenger Duty (APD) doubled on 1 February. Some passengers are having to pay extra despite buying their tickets previously.
Some airlines have said passengers cannot fly unless they have paid the extra cost in advance while others are meeting the charges themselves. Others are letting people pay on arrival at the airport.
Has it caused chaos at UK airports?
There have been no reports of the change causing delays around the UK. BBC correspondents at several main airports have reported some irritation among passengers, but no major problems.
Easyjet, which emailed customers and publicised the changes on its website, said the "vast majority of passengers" with tickets had now paid the difference.
Staff from the airline and Virgin were collecting the additional money in the check-in queues.
Why has this come about?
Chancellor Gordon Brown announced in his pre-budget statement on 6 December that duty on passengers flying out of British airports after 1 February 2007 was to double.
Most airlines raised their flight fares once his announcement was made.
But the duty also applies to those who booked beforehand and this has been dealt with in various ways by the airlines.
The chancellor's critics say it was unusually short notice for an increase in duty and that the announcement has caused confusion.
How much is the new Air Passenger Duty?
The duty has risen to:
- Economy class flights in Europe, 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
How have airlines reacted?
Ryanair says it will not let anyone fly if they have not paid the extra duty by midnight on the day before they travel.
Easyjet gave a deadline of 15 January for payment of the extra cost but extended that to 1 February. Passengers who do not pay by then could be prevented from travelling, it said.
Those who have not received an email can pay direct via the airline's website without incurring a credit card handling fee.
Leeds-based Jet2 said it would contact customers to arrange collection of the additional charge.
Those who cannot be contacted will need to pay at check-in before they travel.
Monarch has been trying to contact all affected customers to arrange payment before travel, or, failing that, at an airport ticket desk.
British Airways has absorbed the tax rise for customers who booked before 12 December, at an estimated cost of £11m to the airline.
Passengers who are unsure whether they are liable are advised to contact their airline before travelling.
What about package holidays?
The vast majority of holidaymakers on a package deal will not be affected, says Abta, which represents travel agencies and tour operators.
This is because when an agency makes any surcharge it is obliged to absorb up to 2%.
Do travellers flying into the UK have to pay the extra tax on their return leg?
Yes, says Michael Carrivick, head of the Board of Airlines Representatives in the UK.
The Air Passenger Duty applies to all passengers whether or not they bought their tickets in the UK, he said.
Are these retrospective charges legal?
Yes, says Simon Calder, travel editor of the Independent newspaper. Terms and conditions of flight bookings mean airlines reserve the right to payment if the government increases the duty, he says.
They contain clauses that mean passengers are obliged to check their account for details of any changes to their flight, and pay the charge.
Why has the government raised the duty and given less than two months' notice?
The Treasury says the aviation industry is not meeting its environmental costs so a decision was taken to introduce the duty increase swiftly.
A spokesman said the chancellor had taken into account economic and social as well as environmental factors.
He said the Air Passenger Duty was not a tax on passengers but a tax on airlines for the number of passengers they carry. It was up to the airline to decide whether they pass that levy on to passengers.
The move would see income from the duty rise from £1bn a year to £2bn, paying for extra resources in public transport and the environment sector, he added.