EasyJet has dealt a blow to Gatwick Airport’s hopes of getting a new runway after it backed Heathrow’s bid and said it would begin flying from the west London hub if it got the go ahead to expand.

A government taskforce is yet to decide which of the two final candidates should be allowed to build the new airstrip, but easyJet’s announcement has swung the argument firmly in Heathrow’s favour.

It will be a particular embarrassment to Gatwick’s owners as easyJet is its biggest customer and currently flies most of its London bound traffic through the Sussex base.

“We can say with great confidence that easyJet would operate from Heathrow if a new runway is built,” the budget airline said in a report.

Carolyn McCall, easyJet’s chief executive, was critical of Gatwick’s proposal, saying there slots were already available at the airport and there was “no evidence” that passenger demand would be expanded by an additional runway.

Relocating to Heathrow would provide 19 new destinations and lower fares, the airline said. EasyJet wants to operate up to 200 flights a day through seven cities including Aberdeen, Edinburgh and Glasgow which are already served by British Airways. A move to Heathrow may raise hope that flights between the facility and Inverness could be restored.

Sir Howard Davies’ commission is taking evidence before it publishes its decision after the General Election. The choice is between an extra runway at Gatwick or more capacity at Heathrow, with either an extended northern runway or a third runway north of the existing pair.