Private firms are being urged to submit plans for a new railway between Heathrow Airport and London Waterloo station without spending taxpayers' money.
Transport Secretary Chris Grayling wants firms outside of public sector body Network Rail to fund eight miles of railway to link the UK's busiest airport and Waterloo.
The project is estimated to cost up to £1.6 billion and the firms involved would earn revenue through fees paid by train operators.
Heathrow, which is planning to build a third runway, has no direct rail connections from the south, with existing lines running east to west.
A private company called Heathrow Southern Railway (HSR) has been set up to promote the project, and several firms are assessing possible options for the route.
HSR says the line could be open by 2025 and would be used by 33,000 passengers per day due to significantly reduced journey times from many locations.
In a statement to the Commons, Mr Grayling wrote: "Governments do not have a monopoly on good ideas for the railways.
"I have been clear that I want the knowledge and expertise of investors and local partners to contribute to delivering new connections, more services and better journeys for passengers."
Mr Grayling has already backed plans for private investment to pay for the reopening of a rail line between Oxford and Cambridge.
He called for proposals on other schemes around the country to be submitted.
Shadow transport secretary Andy McDonald described the Heathrow plan as "optimistic to say the least" given the private sector's "poor record of investing in rail infrastructure", naming the collapse of Carillion as an example.