UK airlines face a downgraded transatlantic open skies deal after Brexit after the US only offered standard access rights.
Washington is balking at maintaining the status quo because airlines such as British Airways and Virgin Atlantic are majority owned by non-UK or US firms.
But UK transport officials insisted last night discussions were progressing well and have made good progress.
The open skies pact allows any EU airline and any US carrier to fly between any point in the European Union and any point in the US.
But the hitch arose when US negotiators offered only a basic bilateral agreement for when Britain leaves the EU.
Standard agreements usually require airlines to be majority owned and controlled by firms from their home country.
British Airways parent International Airline Group and Virgin Atlantic have a mix of ownership, prompting fears flights could be affected.
Negotiators are confident of an eventual agreement to keep open the busy UK-US routes, which account for more than a third of current transatlantic traffic.
But there are legal and political obstacles that could impede the two sides from reaching a deal in time to give legal certainty to airlines booking flights a year in advance, according to the Financial Times.
IAG said: “We have every confidence that the US and UK will sign a deal that is in everyone’s interests and that IAG will comply with the EU and UK ownership and control regulations post Brexit.”
Virgin Atlantic said it remained “assured that a new liberal agreement will be reached, allowing us to keep flying to all of our destinations in North America”.
A Department for Transport spokesman told The Sun: “Our discussions with the US about a new UK-US air service agreement have been positive and we have made significant progress. Both sides want to conclude these discussions soon.
“All parties have a shared interest in ensuring that existing rights will continue under the new bilateral arrangements, allowing airlines on both sides of the Atlantic to continue to operate existing services as well as to seek to develop new ones.”
The US state department said: “Our shared aim with the United Kingdom is to ensure the smoothest possible transition in the transatlantic market.
“Commercial aviation is key to the dynamic economic relationship between the United States and the United Kingdom. Discussions are going well and, while specific dates are not set, we plan to meet again soon.”