Rival airlines Air Canada and WestJet are lowering their fares to $48 for travel between Toronto, Ottawa and Montreal.

The promotion, which must be booked by Monday, follows an aggressive, one-day seat sale announced Wednesday by Calgary-based WestJet. The airline lowered its prices to $17 for travel in the eastern corridor of Toronto, Montreal and Ottawa.

Bob Cummings, WestJet's marketing vice-president, said the airline was offering the reduced fares in a bid to win over new customers.

"WestJet is able to offer fares that are proven to increase our market penetration," Cummings said in a statement Wednesday.

Air Canada spokesman Peter Fitzpatrick said the promotion applies to a limited number of seats.

In April, Air Canada and WestJet offered matching promotions, lowering their prices on select routes to $11 in an attempt to stay competitive and attract travellers.

On Thursday, WestJet said it filled a record 88 per cent of its seats in August. Air Canada, the country's largest airline, said it recorded the second-highest load factor in August with 83.8 per cent of its seats occupied by paying passengers.

Separately on Thursday, Porter Airlines announced it will add six planes to its fleet and triple the size of its Toronto island airport lounge before it expands its service to U.S. destinations in 2008.