John Healey has backed the pro-growth case for a third runway at Heathrow, a stance that puts him at odds with the prime minister, Andy Burnham, who has been carefully treating the issue like a suspicious package since arriving in No 10 in July.

Healey's predecessor as chancellor, Rachel Reeves, had made approval of the long-planned runway the centrepiece of her "growth speech" last year, declaring it would "make Britain the world's best-connected place to do business." Burnham, meanwhile, has suggested the runway should be "a matter for London and Londoners" - a phrase that historians will note is not typically how prime ministers describe nationally significant infrastructure projects.

A recent Sunday Times report suggested Burnham had told the transport secretary, Heidi Alexander, that he was "unconvinced" by arguments for the megaproject. Asked for his views in Bangkok this week, Healey said the government would study responses to a public consultation that recently closed. He then struck a markedly more positive note than Burnham, which, given Burnham's position, was not a high bar.

"I think in recent decades, and this is true in Heathrow and airports right across the UK, that they have been a big part of jobs and growth," Healey said. "And the case for a third runway at Heathrow has always been a case for growth." Healey has insisted economic growth is his and Burnham's "number one mission" in government, which is the kind of thing you say when the number two mission is apparently avoiding the subject.

Heathrow's chief executive, Thomas Woldbye, has warned that investors could reassess their backing for the £33bn infrastructure project if it does not receive the prime minister's support. "Our shareholders would like to do this," he said earlier this week. "They do think that things are taking a long time, and so far they have decided to be OK with that … But of course we cannot wait for ever." That is investor-speak for "we have patience, but it is not infinite, and neither is our goodwill."

The airport's owners include Qatar's investment authority; the French private equity firm Ardian; Saudi Arabia's Public Investment Fund; and Singapore's sovereign wealth fund, GIC. They are expected to decide in the coming months whether to apply for formal planning permission, which is the point at which this stops being a theoretical debate and starts being an actual one.

Healey voted in favour of the third runway when the question was put to the House of Commons in 2018, so his position has the virtue of consistency. The chancellor was in Bangkok to attend the International Monetary Fund annual meetings but has returned to London to put the finishing touches to his 28 October budget.

The UK was praised by the IMF this week for taking action to cut its deficit, while other large economies have opted not to do so. The IMF's managing director, Kristalina Georgieva, urged Healey to "stay the course" in his budget. On Thursday, official figures showed the UK economy unexpectedly continued to grow in August - unexpected being the operative word for anyone who has been paying attention.