Scoot, the proposed low-fare unit of Singapore Airlines Ltd., will start flying in mid-2012 and have a fleet of four Boeing 777-200 planes by the end of that year, as the nation's flag carrier seeks to prevent rivals offering lower fares from chipping away at its market share.

The new airline, with fares as much as 40% cheaper than full-service carriers, will initially fly to destinations in Australia and China. It will add locations in India, Europe and other markets, including Africa and the Middle East, as its fleet expands. 

"No destination is ruled out. Our aim is to develop new routes and hopefully we will do that," Campbell Wilson, the airline's chief executive, said Tuesday at a news conference in Singapore.

The new carrier—which is expected to have a fleet of about 40 planes by the end of the decade—will cater to a different customer segment than its parent or Tiger Airways Holdings Ltd., the Singapore-based short-haul budget carrier in which Singapore Airlines owns a 33% stake.

"We are not a substitute to Singapore Airlines. The aim [for Scoot] is to bring incremental business to the SIA Group," Mr. Wilson said.