Aer Lingus's plans to fly to additional US cities suffered a set-back yesterday when US officials said a draft aviation deal agreed with the Government was unlikely to be honoured without an EU-wide "open skies" agreement.

A US official in Brussels said the administration's clear preference was to have a full "open skies" agreement come into force with all EU member states.

The transitional deal negotiated between Minister for Transport Martin Cullen and his US counterpart Norman Mineta earlier this year was meant to begin working next month. However, the continued failure of EU and US negotiators to agree a wider agreement means this is unlikely to begin operating on time.

Open skies is the name given to a deal that would give EU and US airlines unrestricted freedom to fly between different cities on both continents. Currently, a host of bilateral aviation deals between individual EU member states and the US regulate which airlines can fly transatlantic routes and what cities they can fly into.

The draft deal between Mr Cullen and Mr Mineta would have enabled the Shannon stopover to be phased out over 18 months from November 2006, during which Aer Lingus would have been allowed to fly to eight US airports, three more than at present. After April 2008 Aer Lingus would have been allowed to fly into any US airport enabling it to expand its transatlantic traffic and boost its revenues considerably.

However, at the announcement of the deal this year an US department of transport spokesman stressed that it depended on the conclusion of an EU-US open skies agreement.

Mr Cullen is due to fly into Luxembourg tomorrow to discuss the "open skies" issue with his EU counterparts. He is expected to express impatience with the delay to the talks and suggest Ireland may try to negotiate bilateral arrangements with the US.

A spokeswoman for Mr Cullen said last night the minister was assessing a number of issues. "Our objective is to enable the transitional arrangements to proceed as quickly as possible, failing to do that we will go towards bilateral negotiations," she said.

Despite confidence earlier this year that an EU-US "open skies" deal could be brokered very little progress has been made recently. A key sticking point is the US refusal to enable EU airlines to hold a majority take in an US airline. The US administration has been unable to agree new rules on ownership against a backdrop of domestic pressure from trade unions. Last month British Airways chief executive Willie Walsh warned that it could be two years before a deal can be reached between the two sides.

A failure by Mr Cullen to get the transitional deal approved by the US would be a set-back for Aer Lingus, which is keen to begin operating new routes to the US as soon as possible.

Meanwhile, two Irish MEPs wrote to competition commissioner Nellie Kroes to express concern over Ryanair's bid for Aer Lingus and request that she investigate the matter. Fine Gael MEP Gay Mitchell and Fianna Fail MEP Sean O Neachtain said the deal would hurt competition.