EasyJet said fares would drop by one third with new runway
The airline wants MPs to back the third runway in the upcoming Commons vote.
EasyJet has claimed its air fares would be cut by almost one third if a new runway is built at Heathrow.
The budget airline says passengers would benefit from cheaper flights on existing UK and European routes due to competition.
Expanding the west London airport would allow low-cost carriers to operate a significant number of flights from the airport for the first time, according to easyJet.
EasyJet says it typically offers fares around 30% lower than legacy carriers when it enters an airport.
The Luton-based airline is urging MPs to support the third runway in the forthcoming Commons vote.
It has been working closely with the airport for several years to ensure its requirements will be met if the scheme gets the go-ahead.
This includes 25-minute aircraft turnaround times and enabling passengers to board and disembark using steps at both aircraft doors rather than a single jet bridge, to save time.
EasyJet already operates from other hub airports such as Amsterdam Schiphol and Paris Charles De Gaulle.
Since 2000 the number of UK destinations served by Heathrow has fallen from 14 to eight, while the total number of seats on its flights to Europe has dropped by 40%.
New low-cost entrants to the airport would launch routes to destinations not currently served, easyJet said.
Heathrow's plan to build a third runway was given the support of the Cabinet last week and MPs will soon vote on the scheme.
EasyJet's chief commercial and strategy officer Robert Carey said expanding Heathrow will provide significant benefits to all parts of the UK and is in the best interests of all passengers.
He added: "This expansion would enable low cost airlines like easyJet to operate from Heathrow - in addition to existing London bases - allowing them to provide new routes and increased competition on dozens more UK and European routes.
"EasyJet's costs are significantly lower than legacy airlines so easyJet's fares on these services would be lower than those paid by passengers today.
"We look forward to engaging with the UK's regional airports and their governments and other local organisations to work out which regions will enjoy the largest growth in passenger demand and economic benefits from new connections to Heathrow and the rest of the world."
Mr Carey said expansion must be delivered sustainably, citing its use of Airbus A320neo aircraft which are quieter and use less fuel.