Britain has underestimated the cost of expanding its airport capacity, a UK government commission said today.

Plans to build new runways at either London's Heathrow or Gatwick would cost significantly more than forecast. 

UK lawmakers and business leaders agree Britain needs new runways to remain economically competitive but the idea of building extra capacity in London is unpopular with many voters and a decision has been delayed repeatedly. 

The Airports Commission was set up by the government two years ago.

It has been considering three options of a third runway at Heathrow, an extension to an existing Heathrow runway or a second runway at Gatwick. 

A plan to expand Heathrow could cost between £3-4 billion more than expected, the commission said. 

A second runway at Gatwick airport would also cost £2 billion more than forecast, its added.

The Commission said the higher costs at Heathrow would putt he project at the highest end of the range of financing for infrastructure projects in Britain and said this might push up passenger charges. 

The Commission, which noted that expansion at Heathrow could also result in wider economic benefits, is tasked with making its final proposals on how and where to expand airport capacity by the summer of 2015. 

Heathrow, Britain's largest airport, said October was its busiest on record, with 6.32 million passengers travelling through the airport, up 0.4% from last year. The Commission said it will publish a final report next summer after a consultation.