A young boy is said to have been left in tears after a passenger refused to move seats, after Ryanair double booked it.

The six-year-old's parents paid to reserve a specific seat on the plane from Budapest to Manchester, run by Ryanair's subsidiary Buzz; however, a woman was sitting in it when they boarded the plane.

When they questioned her, she showed them an identical seat reservation and refused to move.

Travel expert Eoghan Corry told The Pat Kenny Show this is very rare.

"Ryanair's response has been that this was a technical glitch and... [it’s] very, very hard to imagine how this happened," he said.

Passengers wait to board a 'redeye' early morning flight at Dublin Airport in May 2010. Picture by: Peter Titmuss / Alamy Stock Photo

The boy's parents are now seeking compensation from the Irish carrier.

"They've been refunded what they paid and the compensation for the stress could end up in a legal process - mostly likely in the English equivalent of the small claims court," Eoghan said.

"Ryanair have already held their hands up and said 'This is something that happens'.

"Anybody listening to us that has a tech system will know that sometimes things come out at the end of your technology that weren't supposed to happen.

"It is very, very rare. I do an awful lot of flights... I think on one occasion I've been given the same seat with a boarding card that somebody else has the exact same boarding card.

"It's a very, very rare occurrence - Ryanair put 3,500 aircrafts into the air every day.

"I don't think it happens very often."

Eoghan said the options in this situation are limited.

"I know lots of people in my life that, if they were on a window seat and they came across a situation like that, would give it up readily.

"Unfortunately, in all of that, when you have to have three people sitting together and a window sit [sic] it was beyond the power of the cabin crew to rectify.

"If you pay for the seat you are entitled - it's on your terms that you give up the seat," he added.