Tens of thousands of airline passengers in the UK are still waiting for reimbursement after their flights were delayed by volcanic ash in April.
Under EU rules, European airlines must pick up the bill for their passengers' hotel and food bills while stranded.
The Air Transport Users Council said most airlines appeared to have huge backlogs of claims to process.
British Airways, the UK's largest airline, has said that it has a large number of claims still to be processed.
A spokeswoman for the airline would not say how large its backlog was, but promised it would be dealt with eventually.
"We have thousands and thousands of displaced customers whose claims we are trying to settle," she said.
"We are doing our very best to get through it, but people will be compensated in due course," she added.
Extra £4,000 costs
James Smith, a computer consultant from Twickenham in south-west London, is one BA passenger who is still waiting for reimbursement.
He flew out to California with his wife Kerry for a holiday on 6 April, expecting to fly home on 17 April.
The return flight was cancelled because of the disruption caused by volcanic ash.
In the end Kerry made her own way back on 21 April with American Airlines - at her own expense - to pick up their young child who had been staying with relatives, but Mr Smith waited until 23 April for the rescheduled flight organised by BA.
Altogether the delays cost the Smiths an extra £4,000. Not all of that is down to BA and Mr Smith is hoping that his insurance policy will pay him about £700 for the delays he and his wife suffered.
But he is asking BA to reimburse him £1,600 for the cost of the extra hotel and food bills which he says it encouraged him to pay, with a promise of repaying him when he returned.
"They said 'make your own arrangements'," Mr Smith said.
"They quoted me on the phone, when I rang one of their helplines, a reimbursement rate of £100 per person per day for accommodation, and between £25 and £30 per person per day for food," he added.
He is also claiming for the cost of travel between hotel and airport.
Now though, back in London for more than a month, Mr Smith is very upset that the process of getting his money has taken so long.
"Despite being assured by BA agents on the phone that I would be reimbursed for accommodation and food, I am having an absolute nightmare getting them to pay up," he said.
"It's impossible to even communicate with them, let alone get any money out of them," he added.
So far he has sent five letters, one a week, by recorded delivery and sent an e-mail every day, asking what has been happening to his claim.
"I have had just a few e-mails back, first of all refusing my original claim, mistaking my claim for reimbursement of the bills with a claim for compensation for delay," Mr Smith said.
"Then they said they would consider my case and I could expect to hear back shortly - that was over two weeks ago and was the last communication I've had from them."
BA acknowledges Mr Smith's extensive correspondence but says the fundamental problem is it has simply been swamped by the sheer number of people claiming under the EU rules.
The UK air navigation service NATS estimates that the six-day close down in April led to more than 38,000 flights of all airlines being cancelled in the UK during that time.
Of those, 4,200 belonged to BA, with tens of thousands of its passengers being inconvenienced.
BA has several departments trying to process the reimbursement claims that have come in, and, as it points out, it is taking longer than usual.
The airline said its ability to process claims had not been able affected by the continuing industrial dispute with cabin crew staff.
"The best idea is to be patient," said the BA spokeswoman. "We are in unprecedented times with the number of passengers being displaced - please bear with us."
BA says it will now contact Mr Smith in the next couple of days to "expedite" his claim.