The Tories say the economic case for a new runway is not proven
Conservative leader David Cameron's decision to oppose the expansion of Heathrow airport has been questioned in a report backed by Lord Heseltine.
Mr Cameron wants to cancel Heathrow's third runway and spend the money instead on high-speed rail.
But a report by Tory think tank the Bow Group says rail versus air is a "false debate" and there was a need for extra capacity at Heathrow.
Lord Heseltine, a key adviser to Mr Cameron, writes the report's foreword.
If elected, the Conservatives have pledged to overturn the government's decision to press ahead with a third Heathrow runway - which could be open by the end of the decade.
Instead, the opposition is backing a high-speed rail link, initially running from London to Birmingham, Manchester and Leeds.
Backing the case for a high-speed rail network, the Bow Group says it is essential it connects directly to Heathrow, rather than through a spur or loop line, to reduce traffic congestion and pollution.
But it argues this cannot substitute for further growth at Heathrow as a new link will actually create demand for more flights from people wanting to take advantage of the better transport links.
Even if all passengers on domestic flights switched to train journeys now, it says growing demand from international passengers mean that Heathrow would still be full within 10 years.
"Many commentators believe that a high-speed rail network is a substitute for extra runway at Heathrow," it argues. "The reality is that they are complementary."
"The Bow Group believes that, by choosing to promote high-speed rail as an alternative to a third runway, the Conservative Party have inadvertently set up a false debate - that the choice is either between either a third runway or high-speed rail.
"This has caused the airline industry to treat the high-speed rail debate in the UK with some suspicion and consequently provide very little input into the government study into linking high-speed rail with Heathrow."
In his foreword, which does not address the runway issue, Lord Heseltine backs high-speed rail as a catalyst for regenerating those parts of the UK "which have fallen behind" economically.
The former deputy prime minister - who advises Tory leader David Cameron on urban policy - argues the choice of the route must not be left to the rail industry as wider social and economic factors must be considered.
The government is also backing the development of high-speed rail and is expected to respond soon to a recent report which set out proposed options for a route eventually running from London to Scotland.