New rules forcing airlines to compensate passengers for delayed and cancelled flights take effect tomorrow, but are being brought in amid confusion and acrimony.
Opponents of the EU regulations say they are unfair to low-cost carriers and are poorly worded.
The EU already faces two legal challenges to the new rules which have been described as "a lawyers' charter".
Under the regulations, airlines whose craft leave from airports in EU member states would have to pay compensation of 250 euros (about £172) a passenger if a flight of up to 935 miles in length was overbooked or cancelled.
The compensation for overbooking or cancellation of flights of more than 1,800 miles would rise to 600 euros (about £416).
The EU regulations also cover compensation for delays of up to two hours for flights of up to 935 miles, as well as three-hour delays to flights from 935 to 1,800 miles and for four-hour delays to flights of more than 1,800 miles.
The compensation would include free meals, refreshments, free phone calls and hotel accommodation if the delay continued overnight. Airlines failing to comply could face fines of up to £5,000 a passenger.
But there is confusion about what constitutes a delay. Airlines are exempt if the cancellation is caused by extraordinary circumstances which could not have been avoided, but this appears not to cover technical problems with the aircraft.
One low-cost carrier which is particularly unhappy with the regulations is easyJet. "We think this is poor legislation. There will be plenty of arguments about it and the only people who are going to get rich from this are the lawyers," an easyJet spokesman said today.
He added: "One of the legal challenges concerns the fact that the whole compensation package is disproportionate and is not based as a percentage of the fare paid.
"However, we can assure customers that fares will not go up as a result of this. They are going down and will continue to go down."
The UK government opposed the regulations when they were first proposed. The Department for Transport said today: "The UK supports the rights of air passengers and welcomes the long overdue updating of the existing regulation on denied boarding compensation.
"However, we are mindful of industry concerns about the proportional impact of this legislation, particularly on the no-frills sector, and will carefully monitor the situation over the coming months."
