Fresh doubts have been raised about the Heathrow Airport expansion amid claims Andy Burnham highlighted his previous opposition in meetings with MPs.
The PM is said to have told Labour colleagues that he had 'never been in favour' of the scheme to build a third runway.
The reported comments came despite Chancellor John Healey telling the Commons earlier this month that the Government still supports the project.
Mr Burnham has a record of attacks on the proposal going back years. In January last year the then-Manchester mayor branded it a 'model for an ever-overheating UK economy', warning it could not come at the 'expense of essential investment in northern infrastructure'.
In 2018 he said: 'I'm against a third runway at Heathrow because of the need to limit national levels of aviation We should use spare capacity at Manchester and Birmingham instead.'
The following year he warned that the expansion would 'trap infrastructure spending inside the M25 for 10 years'.
Andy Burnham is said to have told Labour colleagues that he had 'never been in favour' of the scheme
Heathrow Airport bosses want to obtain permission to build a full-length, 3,500-metre runway (artist's impression)
A Labour MP told the Financial Times that Mr Burnham had referred to his views in a recent private meeting.
'He said he's never been in favour of Heathrow expansion, but he's not indicated one way or another what he will do now,' they said.
'He's indicated that in the past he has never supported Heathrow's expansion... I get the impression that he's now waiting for a report into the third runway by the Transport Committee before saying more on this.'
Government sources played down the significance of the reported remarks. Mr Burnham has made a series of U-turns on former views since taking power.
Heathrow Airport Limited wants to obtain permission to build a full-length, 3,500-metre runway, which would require the M25 motorway being moved.
The plans to expand the UK's largest airport have long been opposed by climate experts and Mayor of London Lord Khan.
But Heathrow insists the project will create jobs, unlock growth and make travel more affordable.
John Healey told the Commons earlier this month that Labour backs the project despite the change of PM
In June former chancellor Rachel Reeves said she was determined to get 'spades in the ground' for the third runway in the current Parliament, and for it to be built by 2035.
Her successor Mr Healey told MPs earlier this month: 'After years of delay, years of indecision, years of often vexatious challenge, we have given the go-ahead to this project quite simply because the economic case stacks up and will help bring growth.'
A public consultation on the Government's Heathrow expansion national policy statement closed on September 1.