The Conservative Party looks set to make airport expansion a major issue at the next election after it followed its commitment to scrap plans for a third runway at Heathrow in favour of a new high-speed rail link, by yesterday confirming it would also oppose plans for expansion at Stansted.

Speaking at a press conference to announce the move, shadow transport secretary Theresa Villiers urged airport operator BAA to scrap a £160m planning application for a second runway at Stansted, warning that a Tory government would not grant approval for the plan.

BAA's planning application will enter its consultation phase next year with a final decision likely in 2010 – around the time of the next general election.

She also warned that even if the government grants approval for a third runway at Heathrow, a Conservative government would attempt to rescind the decision.

"We will not consider ourselves bound by any decision taken by this government," she said, adding that as a result any firm "getting involved in any contract for a third runway should be very, very careful".

The comments appear to be part of a concerted effort to bolster the Conservatives' green credentials and put clear distance between themselves and the government over airport expansion.

Last month, the party unveiled plans for a £15.7bn high-speed rail link between London, Birmingham, Manchester and Leeds that it claims would reduce demand for domestic flights from London airports, limiting the need for new runways.

In contrast, transport secretary Geoff Hoon last week granted approval to changes in planning conditions that will allow BAA to increase the number of flights to and from Stansted from 241,000 to 264,000, while observers are increasingly convinced that the government will force through plans for a third runway at Heathrow when it finally reaches a decision later this year.

However, it remains unclear whether the Tories are seeking to block all airport expansion or simply seeking to put the brakes on expansion at existing sites.

Last month, Conservative London Mayor Boris Johnson announced plans for a feasibility study into a new airport in the Thames Estuary to replace Heathrow.

But a spokesman for the Conservative Party said that the proposals did not represent Tory policy, adding that the feasibility study was commissioned by the mayor and the viability of a new London airport not being investigated the Conservatives.