The UK's aviation regulator says concerns over the cost and schedule of a new Heathrow Airport runway must be answered "urgently and decisively".
In a letter to the Department for Transport, the Civil Aviation Authority says it has concerns over Heathrow Airport Limited's "information flow".
Airlines have tried and failed to get answers to their concerns, CAA chief executive Richard Moriarty writes.
Answers are needed to retain confidence in the third runway project, he says.
Plans to build more airport capacity in the south east of England have been beset by rows, delays, protests and competing plans.
But in June the Cabinet finally approved a new runway plan for Heathrow, a move described by ministers as a "historic moment" for the UK.
Lack of information
The Department for Transport (DfT) asked the Civil Aviation Authority (CAA) to monitor "how well Heathrow Airport Limited (HAL) is engaging" with interested parties over the £14bn-plus expansion.
In his interim update to the DfT, Mr Moriarty acknowledges the complexity of the project, but says there are two important areas of concern - "high quality information" about costs and a revised timetable.
Airlines, worried about increased charges to use Britain's biggest airport, have been complaining for at least 18 months about a lack of information, he says.
"These concerns have not yet been adequately addressed despite repeated requests from the airline community and the CAA," the letter says.
Specifically, Mr Moriarty wants more information about how HAL can keep a promise to hold airport charges close to current levels while ensuring a third runway is commercially viable.
He points out that unless more details are disclosed, the CAA has legal powers to force the airport's owners to reveal information.
But Heathrow insisted everything was on track for the runway to be in use in 2026.
A spokesperson said: "We continue to engage with all of our stakeholders on our expansion plans and look forward to presenting a detailed preferred masterplan for further public consultation next year.
"We remain on track to submit a planning application in 2020 and for the new runway to open in 2026."
Airlines said it was only right that the CAA press Heathrow for detailed clarity about the massive infrastructure project.
IAG, parent company of the airport's biggest airline, British Airways, said in a statement: "Heathrow is the most expensive hub airport in the world. It's outrageous that it has yet to provide a detailed breakdown of the £14bn expansion costs.
"The CAA is right to demand this as the regulator needs to ensure that passenger charges don't rise from current levels and the UK benefits from cost effective infrastructure so that it can compete on a global scale post Brexit."