Passengers are being warned of possible chaos at airports next month as some airlines seek to charge extra air tax for tickets bought months ago.
Abta says extra payments should be collected at check-in
From 1 February the Air Passenger Duty will go up by between £5 and £40.
Most airlines raised fares as soon as Chancellor Gordon Brown announced the hike on 6 December.
But some are demanding further payment for tickets booked before the rise. Easyjet said the situation was "absolutely chaotic".
The chancellor's critics say two months was too short notice for the duty increase and it is unfair to charge customers who booked months earlier but will fly after 1 February.
Many airlines say they will make a loss if they do not charge passengers the extra duty.
But not all agree how they should charge, causing confusion for travellers.
DUTIES FROM FEBRUARY
Economy class in Europe £10
Business and first class in Europe £20
Economy class on long-haul flights £40
Business and first class on long-haul flights £80
Ryanair chief executive Michael O'Leary said all passengers will be e-mailed and if they have not paid by the day before their flight, they will not be able to fly.
Leeds-based Jet2 says it is in the process of contacting customers to arrange collection of the additional charge.
British Airways says it will absorb the costs of duty on passengers who booked before the announcement.
Toby Nichol, of Easyjet, said: "It's absolutely chaotic. Come 1 February, there is going to be a lot of uncertainty.
"There will be an awful lot of airlines walking up and down check-in aisles in places like Luton Airport, with a bucket collecting the tax."
Roger Wiltshire, of the British Air Transport Association, said plenty of notice would usually be given to duty changes.
"It's a mess because the chancellor brought this in at such short notice," he told BBC's Five Live Breakfast.
"They have caught millions of passengers who had booked in advance and paid already."
But the Treasury says the aviation industry was not meeting its environmental responsibilities so a decision was taken to introduce the duty increase swiftly.
It says the extra £1bn in duty will go towards better public transport and the environment.
A spokesman said the Air Passenger Duty was a tax on airlines for the number of passengers they carry and it was up to the airline to decide whether they pass that levy on to passengers.
Sean Tipton, of the Association of British Travel Agents (Abta), said it would have preferred the duty rise to be postponed until the summer but airlines should be collecting the extra payment at check-in.
He advised passengers who booked before 6 December 2006 to check with their airline whether payment was due before travelling.
James McLean, of Allseasons Travel in Hereford, said despite Easyjet demanding payment by Monday, the budget airline had sent emails to only one out of five of their customers.
"We have had to chase Easyjet. It's time and it's also our phone bill," he said.
The rise will mean for example that tax on a short-haul flight in Europe, including internal UK flights, will double to £10 per passenger and to £80 per passenger on long haul business flights.
Simon Calder, travel editor of the Independent, said that while the duty rise had been expected, the liability of those who had already booked had not.
For a family of four travelling between Leeds and Belfast that could mean an extra £40 which could make some people's account overdrawn, he said.