More than 2,000 British Airways passengers have had their tickets cancelled because the prices were too cheap, a travel agent has said.
Travel Up chief Ali Shah said he believes another five agencies also sold tickets through the airline's mistake.
BA has apologised for the error on flights to Tel Aviv and Dubai but refused to say how many were affected.
Customers said they were angry their tickets are not being honoured.
Mr Shah said flights normally costing more than £200 were advertised for £1 plus airport taxes, which can be several hundred pounds, between 17:45 BST on 11 June and 11:00 the following day.
The agent, who said more than 2,000 of his customers have been affected, said: "It's very cruel for the customers because they have booked these flights in good faith and expected it to be honoured.
"But I can also understand British Airways' position, it's a human error."
Mr Shah said all affected customers are being contacted and BA will provide a full refund as well as a £100 voucher.
Travel Up customer Ash Gubbay, from London, says he had booked a return flight to Tel Aviv for £195 but now has to pay about £1,000.
Mr Gubbay said he thought the tickets were cheap but "not far-fetched" and comparable with other airlines.
He said the firm should honour the flights.
Mr Gubbay said he was "annoyed and angry" and the voucher would be of "no help" as it will not cover the cost difference for new flights.
He said: "I'm very disappointed and let down by British Airways.
"If I wanted to cancel my tickets I wouldn't have been able to but it seems like they can just do what they want."
Another customer, Esther Vadia, had booked six tickets to Tel Aviv for £167 each and a seventh for her son later in the year for £195.
'Not excessively cheap'
All have been cancelled.
She says she is unable to afford to pay for other flights and although she will be refunded, she will still have to pay credit card fees for the cancelled tickets.
Ms Vadia also said she would end up losing more than £1,000 on non-refundable accommodation in Israel.
She said: "If you go to a supermarket and buy something for cheap, they cannot come after you later and say you have to give it back.
"So how can an airline do that? Besides, I don't think the tickets were so cheap as to be unbelievable."
Was BA within its rights to cancel the tickets?
by Brian Milligan, BBC personal finance reporter
Whether BA was entitled to cancel the tickets is likely to depend whether they were so cheap that the so-called mistake should have been obvious to customers.
If they were miles cheaper than comparable tickets, the company could argue that it made a "unilateral mistake" in its pricing.
But the difficulty for BA is that a quick search online reveals flights from London to Tel Aviv to be on sale for around £215. So the BA fare of £195 may not seem unusually good value.
James Daley, managing director of Fairer Finance, thinks BA may be in the wrong.
"I don't understand how, technically, they could wriggle out of it. It's their error," he said.
However, it may take a legal challenge from one of the passengers involved to fully test the law on this.
Hanna Simon, another passenger, also said she did not believe the flights were "excessively cheap".
She said she was particularly frustrated as she had corresponded with BA about add-ons to her flight and no problem had been raised by the airline.
Several people the BBC spoke to also said they had not been informed of the cancellation and only found out when they checked their online accounts.
They also said the £100 voucher was "useless" as, aside from "restrictive" terms and conditions, the flights were now costing much more than that.
BA said in a statement: "Errors like this are exceptionally rare, and if they do occur, under contract law, there is no binding contract between the parties.
"We have apologised to customers and offered a gesture of goodwill."
Guy Anker, deputy editor of MoneySavingExpert.com, said BA should "do the right thing" and let the passengers "fly at the price they booked".
He said: "While this may have been a 'rare' error, it's not the passengers' error.
"They've bought these tickets in good faith at a believable price - it's not as though the tickets cost £5, which would clearly have been a glitch."
Jack Sheldon, founder of Jack's Flight Club, said "fare errors" were not uncommon but in almost 70% of cases the airline would honour the tickets.
He said: "They honour it because it's bad PR to cancel tickets.
"This is an odd one. In cases where tickets are cancelled it is usually because the ticket was supposed to be say £400 but was sold for £40.
"In this case the error doesn't seem to have been that big and a typical customer could well have thought this was a genuine fare."