Chancellor John Healey has insisted that the government backs a third runway at Heathrow, as the airport warns that investors could abandon the £33 billion scheme unless ministers give final approval by December.

The assurance leaves a crucial distinction unresolved: government support for expansion is not yet the final decision Heathrow’s investors are seeking. The Department for Transport is still assessing consultation responses and updated information on the project’s benefits.

Appearing before the Treasury Select Committee on Wednesday, Healey said: “So the Government is behind a third runway at Heathrow, I’m part of the Government.”

He argued that successful airports often supported growth in their surrounding areas, citing Manchester as well as London. He expressed frustration at the time Britain takes to approve and build major infrastructure, saying the government had sought to accelerate the process over the past couple of years.

That economic argument sits alongside an earlier, more cautious position. Healey opposed the project as Mayor of Greater Manchester. In September, he said: “I think principally, it's a matter for London and Londoners.” He added that their voices should be heard first and that it was not right to intervene immediately after the consultation.

The immediate pressure comes from Heathrow’s financing and timetable. Earlier this year, the airport warned that planning and regulatory issues could delay the runway by four years.

Chief executive Thomas Woldbye has set December as the deadline for a government green light. He warned that investors, including sovereign wealth funds from Saudi Arabia, Qatar, Singapore and China, would withdraw without final reassurance that the scheme would proceed.

Ministers therefore face two different demands: completing the assessment of expansion’s benefits and providing the certainty Heathrow says is needed to retain its investors. Healey’s endorsement does not, by itself, settle either.