High-speed rail services could reduce the number of flights from Heathrow
High-speed rail links would benefit the environment and the economy more than plans to expand Heathrow airport, says a survey by the UK's rail union.
The Rail, Maritime and Transport (RMT) union said switching investment from airports to tracks would create jobs and cut pollution.
The RMT's report said that if high-speed rail links were introduced, Heathrow would not need a third runway.
It warns the UK may be left behind as European rivals boost rail investment.
The RMT's report found that more than a third of all flights from Heathrow were short-haul and more than 20% of those went to places that could be reached by rail.
It also found that commuters were willing to switch from air to rail where high-speed links existed.
The UK was also in danger of falling behind European countries such as Spain, where the government had announced a significant rail investment, the report said.
RMT general secretary Bob Crow said: "Paris remains Heathrow's top destination and there are as many flights leaving for Edinburgh as for New York.
"If we provide a viable, fast and sustainable alternative to short-haul flights, the case for Heathrow expansion would evaporate."
Last November, the government set out proposals for a third runway and a sixth terminal at Heathrow by 2020, raising concerns about safety and pollution.
Environmental pressure groups have argued that building better rail services would persuade travellers to abandon cars and planes.
The case for modernising UK rail services was highlighted over the weekend by a review from the organisation that controls Britain's mainline railways.
Network Rail said it would look at the prospect of building five new high-speed lines crossing the length and breadth of the UK to cope with increasing passenger demand.
The review would also assess the need for high-speed trains, similar to the French TGV, to cope with the rising number of rail users.
In the past decade, passenger numbers have risen by about 40%.
But the UK has fallen behind its Continental rivals. In France, Germany and Spain, 300km/h train services are common.