Transport for London (TfL) has criticised the government for failing to share key information on Heathrow expansion.
Speaking at a London Assembly Transport Committee meeting on Thursday, Alex Williams, acting managing director of planning at TfL, said: "There is an ongoing issue that pre any legal challenge, we have asked for a regular series of updates of information on modelling and air quality issues and it just never gets supplied, so we would be very keen to share information."
London mayor Sadiq Khan has said he will back a legal challenge against the airport expansion and directed (TfL) to provide advice and assistance to affected borough councils. TfL will be named as an interested party. But Williams said the problem hadn't just emerged since the mayor's announcement last month.
Williams said: "In terms of exchange of actual information it's just not happening and that's not recent; that's a longstanding issue."
Val Shawcross, deputy mayor of London for transport, said: "Whatever political position we take in opposition to this Heathrow expansion and our aspiration elsewhere i.e. Gatwick, we would not stand in the way of technical developments, information exchange and debate about the surface transport needs."
She added that "it would be tempting to say let's block that conversation in order to try and slow the project", but the mayor's team would not "in any way" seek to stymie any technical discussion of the knock-on impact in terms of air quality and noise.
As a result of its involvement in the legal challenge, TfL and City Hall have been informally notified they won't be sitting on a Department for Transport (DfT) group discussing surface access.
TfL has butted heads with transport secretary Chris Grayling over predicted costs for improving rail and road links for Heathrow expansion - Grayling recently said TfL's estimates of £15bn for surface infrastructure changes were "ludicrous".
But TfL stuck by its analysis and said a key component was the delivery of a new southern rail link from Waterloo to Heathrow. A spokesperson warned: "Thus far, the government has given no commitments to deliver this new rail link, despite the Airports Commission's recommendation to do so and, without such a commitment, the aspirations for no increase in road traffic are not credible.
And Shawcross said today that TfL had "a long and successful track record" in making such forecasts. "So this is not pie in the sky and they can't be dismissed lightly as I think they recently have been."
A DfT spokesman said: “We have regular meetings with TfL on airport surface access issues and these will continue in the future.
“We recognise TfL will have an important role in making surface access to the airport work.”