Britain's opposition Conservatives would scrap a proposed third runway at Heathrow airport and instead build a new high-speed rail line between London and northern England, a party official said.

Conservative transport spokeswoman Theresa Villiers said the £20 billion railway would cut 66,500 flights a year, some 44 per cent of the capacity of the proposed new runway.

"Evidence from right around Europe clearly shows that high-speed rail provides a viable and attractive alternative to short haul flights... And it could deliver this without the environmental penalties of a third runway," she told the party conference in Birmingham.

She said a new high-speed rail network could add £60 billion pounds to Britain's economy, and boost transport links between the north of the country and the south.

Opinion polls suggest the centre-right Conservatives are on course to return to power at the next election, due within 18 months.

Conservative leader David Cameron has already questioned the case for a third runway at Heathrow on environmental grounds, upsetting business leaders who say the economy will suffer if the airport is not enlarged.

The business-focused Confederation of British Industry (CBI) said a new rail link would not be enough to offset a refusal to expand Heathrow.

"A high-speed rail link would have a lot going for it, but don't think for a minute that it will solve the capacity problems at Heathrow," CBI Director-General Richard Lambert said in a statement.

Prime Minister Gordon Brown's Labour government is considering its response to a public consultation on expanding the airport, the world's busiest international hub with 68 million passengers passing through each year.

Outgoing Transport Secretary Ruth Kelly, who resigned last week, said the Conservatives' position was a "false choice" as extra capacity was needed at both Britain's airports and railways - a stance supported by Heathrow owner BAA.

"These proposals are politically opportunistic, economically illiterate and hugely damaging to Britain's national interests," Ms Kelly said in a statement.

The construction would involve the demolition of the village of Sipson, including around 700 homes.

The Conservative plans for a railway would link London, Birmingham, Manchester and Leeds, as well as connecting to the Eurostar service to Paris - itself made faster last year.

A Conservative government would spend £15.6 billion to build the new line between 2015 and 2027, with the remaining £4.4 billion to come from the private sector.