British budget airline easyJet has backed Heathrow in London's runway expansion race and said it would start flying from the capital's largest airport if the government chooses to increase its capacity.

British lawmakers and business leaders agree Britain needs new runways to remain economically competitive. A government-commissioned taskforce is assessing whether a new or expanded runway should be built at Heathrow, or Gatwick south of the capital.

"We can say with great confidence that easyJet would operate from Heathrow if a new runway is built," the airline said in a report submitted to the taskforce as part of a consultation.

The airline, based at the smaller Luton Airport north of London, does not offer flights from Heathrow which is typically more expensive to operate from.

Potential easyJet flights from Heathrow would provide 19 new destinations from the airport and lower fares, the airline said, adding that its tickets are typically 40 per cent cheaper than those of incumbent operators.

EasyJet said expanding Heathrow airport would provide the greatest benefits for passengers and Britain's economy.

Heathrow Airport said it was "delighted" by the news, while Gatwick Airport slammed easyJet's backing of Heathrow as a decision based on "its own narrow commercial interests".

Gatwick said its project would cause less environmental damage and fewer people would be affected by noise.

Airport expansion plans have been unpopular with voters who are concerned about increased noise and traffic on homes in the London area and the government has repeatedly delayed a decision on the matter.

The government's Airports Commission also found in November that project costs had been underestimated, with Heathrow's expansion costing between 3-4 billion pounds. – Reuters