A computer error meant around 900 business class seats on Singapore Airlines were sold at economy prices earlier this week.
The airline is now telling agents they need to get passengers to pay the difference or give the tickets back.
Fares for Singapore flights to various European airports were incorrectly loaded into the system between Saturday and Monday.
Some agents picked up on the error and contacted the airline immediately, but around 900 tickets had already been sold.
A spokeswoman for Singapore Airlines confirmed a mistake had been made, but said the airfare conditions stated it was only valid for economy class travel.
"The fare issue was discovered last weekend and action was to taken to rectify the issue immediately," she said.
"We have advised our travel agents of various options available to customers, including paying the difference to our lowest business class fare, travelling in economy class on the booked ticket, refunding the difference between the fare charged and our lead in economy class fares, or offering a full refund.
"We are working with our travel agent partners to ensure that our customers are notified of this issue, to ensure there is no disruption to their travel plans.
"Singapore Airlines would like to apologise for any inconvenience caused."
