Continental Airlines, the No. 4 US airline, will charge customers for seats with extra legroom on its flights worldwide starting on March 17.

The cost of this extra space will vary depending on a slew of factors such as the length of a flight and competition from other airlines within a market, said spokeswoman Julie King. The date and time of a flight can also alter the prices for roomier seats, Ms King said.

Travellers can get seven extra inches of legroom on regional jets and up to 31 extra inches on larger planes, Ms King said.

The option to purchase the extra legroom would be available when passengers check in for their flights. Continental passengers can check in 24 hours ahead of departure. Members of Continental's frequent flier program can still book these seats at no additional charge, Ms King said. Platinum and gold members of Continental's OnePass Elite program can book these seats when they make a reservation.

Platinum members typically fly 75,000 miles a year, while gold members fly 50,000 miles a year. Silver members, who fly about 25,000 miles a year, can book these seats at check-in.