Singapore Airlines (SIA) has introduced a new bidding system for seat upgrades which will be available to invited customers, the airline said this week.

Customers who have booked tickets in economy class can get an invitation to bid for an upgrade to premium economy class.

The new system, called mySQupgrade, will be "gradually introduced across the network", said SIA in an e-mail to members of its loyalty programme, Krisflyer.

Such bidding systems have been adopted by more airlines in recent years. More than 30 airlines around the world offer these "auctions", The Wall Street Journal reported in January.

SIA subsidiary Scoot already offers the bidding service for upgrades to business class, while Tigerair lets travellers bid for empty middle seats to ensure that no one sits beside them on the flight.

How the upgrades are offered depends on the airline. Some may prioritise frequent fliers, others may offer them to passengers travelling alone.

SIA said it will notify eligible customers seven days before their departure. Bids must lie within a "preset range", it said.

Travellers can change their bids up to 50 hours before departure. Payment will be made only if the bid is successful, and successful bidders will be informed 40 to 48 hours before their flight.

"Invitations are made at Singapore Airlines' sole discretion based on certain criteria and combination of factors, which are dynamic," the airline said on its website.

SIA also said that while only one passenger in a group may receive an invitation, these guests can submit a bid on behalf of other passengers with whom they are travelling.

Factors that affect the outcome of a bid can include the value of the bid, the value of other bids, and the number of premium economy seats available, SIA said. It will prioritise redemption booking confirmations over upgrade bids through mySQupgrade.

Premium economy seats, introduced by SIA in August last year, come with more space, individual in-seat power supply, two USB ports and extra stowage space.