Not building a third runway at Heathrow Airport would be a "huge missed opportunity" for the UK, the boss of the country's busiest airport has warned.
Thomas Woldbye told BBC Radio 4's Today programme it would be "good to know" the government's position on the project, after Prime Minister Andy Burnham refused to say whether he backed expansion.
The airport's chief executive said if its owners could not invest in a third runway, the money "will be invested somewhere else in the world", suggesting a decision would be made in the new year.
The Department for Transport said it was reviewing responses to a consultation and was "considering the appropriate next steps".
A spokesperson added that the government supported airport expansion "across the country where this can contribute to growth and opportunity, while meeting the tests on the environment".
They said expanding Heathrow had the "potential to deliver around £40 billion in economic benefits across the country".
Building a third runway at Heathrow has been debated for decades. The project has proved controversial, facing steep opposition from local residents, politicians and climate campaigners.
Last year, then-chancellor Rachel Reeves announced the government-backed expansion, saying that "a third runway is badly needed", and that the government could not "duck the decision any longer".
Heathrow Airport is owned by a consortium of investors, the largest of which is French private equity firm Ardian. Others include the Qatar Investment Authority, Saudi Arabia's Public Investment Fund and the China Investment Corporation.
"If they cannot invest in Heathrow... then that money will be invested somewhere else in the world," Woldbye said.
"It's not about allocating this money elsewhere. It's private money. It's not public money. So hence, it will not go into the UK. I think that would be really a sad, huge missed opportunity for this country."
A third runway would mean demolishing hundreds of homes, diverting rivers, and rerouting the M25 motorway between junctions 14 and 15 through a tunnel under the new runway. It is estimated it would cost £33bn to build.
Heathrow serves more than 80 million travellers a year with its four passenger terminals and two runways. The number of flights, currently capped at 480,000 a year, could go up to 720,000 - or nearly 2,000 a day on average, if a third runway is built.
This additional traffic would potentially lead to a huge surge in revenue for the airport's owners, and Heathrow has argued the expansion would provide a "huge economic boost" and create jobs.
The new runway was scheduled to open by 2035, subject to plans being submitted and approved, but Heathrow has since suggested it would be delayed by four years to 2039 due to planning and regulatory issues.
Whether the new-look government still backs a third runway is not yet clear. Burnham, who as Mayor of Greater Manchester opposed the scheme, said he believed the matter was "for London and Londoners".
"I think we ought to hear the voice of people there first. It's not right for me to just jump straight in on the back of the consultation," he said, speaking to the BBC last month.
Woldbye said he hoped the prime minister would "will come along" with the project, pointing out it was backed by the unions.
Woldbye told the Today programme he had not yet met the current Chancellor John Healey, but if he did, he would outline that the airport "contributes dramatically to the economy" but that "we are now full".
"That means we will stop contributing or stop growing that contribution. If we want to avoid that, we need to now take this decision, move forward and continue the work of the previous government to move at speed and deliver this," he said.
The Department for Transport said it was still finalising its Airports National Policy Statement, which provides the basis for deciding whether to grant a new runway at Heathrow. It was last updated in 2018.
Woldbye said Heathrow was working on a planning application which has planned to submit in the next couple of years, but said any decision would be guided by government's the national policy statement.