A transatlantic air deal that could lead to lower fares for passengers heading for America was finally agreed yesterday.

But the long-awaited "open skies" agreement between the European Union and the US to liberalise travel between the member states and America is not quite the laissez-faire arrangement it claims to be.

The deal means any EU or US airline will be allowed to fly between Heathrow and America rather than just the four carriers, British Airways, Virgin Atlantic, United Airlines and American Airlines, permitted to do so at present.

But critics are angry over what they see as the one- sidedness of the deal which gives American carriers more freedom to fly in the EU than European airlines will have in the US.

This lack of reciprocation had led to opposition to the agreement at Westminster but, yesterday, Transport Secretary Douglas Alexander joined 26 other EU Transport Ministers to agree the deal unanimously.

However, government concerns remain and Mr Alexander, MP for Paisley and Renfrewshire South, won a key concession. The ministers agreed the EU would scrap the deal within two years unless Washington agrees to allow EU airlines to fly between US cities and to allow greater foreign investment in US carriers.

He also obtained a delay in any start date from October to March 30 next year, by which time Heathrow's new Terminal 5 will be operational.

As well as allowing more airlines to use Heathrow for transatlantic flights, the agreement will permit European airlines for the first time to run transatlantic flights to any American city from any EU country. In return, American carriers will have free access to European airports.

Mr Alexander said the deal would bring new opportunities for UK airlines and new benefits for UK passengers but the Conservatives accused the government of "caving in" to the Americans.

Willie Walsh, chief executive of BA, said nothing less than a complete open-skies deal was acceptable, while Sir Richard Branson, chairman of Virgin Atlantic, said it was "a good day for the traveller".

The UK's Air Transport Users Council said passengers should not expect to see an immediate dramatic dip in the price of fares.

Airlines wanting to fly to the US will first have to get take-off and landing slots at Heathrow, which are limited by the size of the airport. Mr Walsh said BA would not be giving up any slots and he argued transatlantic fares were already low, and competition fierce.

However, Mr Alexander said the deal delivered real benefits for UK consumers of up to £250m a year.

Julian Brazier, Shadow Aviation Minister, said: "Everybody is in favour of a genuine open-skies agreement but that is not what this deal is."

Simon Evans, chief executive of the Air Transport Users Council, said: "This agreement is good news for air passengers and the airline industry but I don't think we're going to see a tremendous drop in the price of tickets."

Jack Dromey, deputy general secretary of the Transport and General Workers Union, said: "No other country in Europe has an airport of the size or significance of Heathrow. It was predictable but sad, therefore, that the EU did not stand up to the Americans."