The US government has warned that it will take action to block a merger between British Airways and KLM, unless it achieves a breakthrough in open skies negotiations with the UK.

"The US will not allow British Airways to use a merger with KLM, the Dutch flag carrier, as a back door to achieve greater access to our market," said Ms Dorothy Robyn, a senior White House official.

Washington made clear that KLM, the Dutch flag carrier, would jeopardise its unrestricted access for flights into the US if it were to pass into British ownership without prior agreement on liberalised air services between the UK and the US.

Ms Robyn, a special assistant to President Clinton said: "If KLM comes under effective control of British Airways while Bermuda II still governs US-UK air services, KLM will immediately lose the benefits of the US-Netherlands open skies agreement."

Bermuda II, the US-UK bilateral air services treaty which dates back to 1977, is one of the most restrictive in international aviation.

Under the bilateral treaties that govern international aviation, an airline's route rights are tied to the identity of its national ownership.

One model that has been under consideration by the airlines' bankers would give BA economic control but leave majority voting rights in Dutch hands to allow the appearance of continuing Dutch national ownership.

The US is seeking to use its power to block a BA-KLM deal as the lever to secure open, or greatly increased, access for US carriers into London's Heathrow Airport. Ms Robyn said access to US routes in Heathrow was limited "to a small club of airlines" - BA, Virgin Atlantic, and United Airlines and American Airlines of the US.