SEATTLE is a popular destination for British business travellers. Home to Micro-soft, Boeing’s commercial-aircraft operations and a host of other high-tech companies, it draws investors, entrepreneurs and salesmen from across Britain.

If you want to fly straight there from Heathrow, however, you only have a choice of one airline, British Airways. BA has the only direct service from London’s premier airport, and if you want to fly in business class to Seattle next week, it will cost you, if you book online with BA, nearly £6,000. If you want to pay less, you will have to take a circumlocutory route with a US carrier via one of the big American hubs, changing planes in, for example, Chicago.

These sky-high prices are the legacy of BA’s protected position at Heathrow. Thanks to a 30-year-old treaty between the US and Britain, only four airlines – BA and Virgin from the UK, and United and American from the US – are allowed to fly from Heathrow to America.

All that changes in April. Thanks to a new open-skies treaty between Europe and the US, Heathrow is being thrown open to competition. On Wednesday – as The Sunday Times revealed last weekend – Air France and Delta Air Lines, two of the world’s biggest airlines, declared war on BA by announcing plans to join forces to mount transatlantic flights from the London hub. According to airline industry insiders, this is just the beginning of a frantic jostling for position at Heathrow, a struggle that will reshape transatlantic flying, may lower fares, and could eventually determine which airlines emerge as dominant players globally.

The initial struggle at Heathrow is between airline “alliances”, groups of airlines that have joined together for marketing purposes. Alliances have grown up because ownership rules have stopped international airlines buying one another, leading to de facto integration through marketing tactics such as code sharing – where one airline’s code is put on another’s flight.

Air France and Delta are part of the Skyteam alliance. The pair have been granted immunity from American antitrust laws, meaning that they can in essence run their planned Heathrow operation as a joint venture, colluding on everything from fares and capacity to schedules. They plan to fly from Heathrow to New York, Atlanta and Los Angeles.

On its own, Delta would have struggled to break in to Heathrow. The airport is full and landing and take-off slots difficult to acquire, and pricey – between £5m and £10m per pair. But Delta got by with a little help from its friends.

Air France is cutting back its London-Paris flights, and giving those slots over to the transatlantic operation.

Holland’s KLM and America’s Northwest Airlines are also part of Skyteam. They are expected to join the party at Heathrow, with Northwest likely eventually to start flights to Seattle, which is one of its US strongholds. Skyteam has asked Wash-ington for blanket antitrust immunity for all four airlines. It is expected to be granted in the middle of next year.

BA is a founder of the Oneworld alliance, which has American Airlines as its US member. BA and American have tried for antitrust immunity in the past, but have been put off by the demands from competition authorities on both sides of the Atlantic. The regulators insisted that BA’s dominance on the North Atlantic was such that it would have to surrender prized landing slots at Heathrow before they would grant immunity – too high a price for the British airline to pay. BA and American are, however, likely to make a fresh application in the future, once the impact of open-skies has become apparent.

The final player is Star, the alliance founded by Germany’s Lufthansa and America’s United, and which now includes BMI British Midland, the UK airline chaired by Sir Michael Bishop.

While Star has been slow out of the blocks compared with Skyteam, BMI is the potential ace up its sleeve. BMI holds 13% of all Heathrow slots, second only to BA, which has just over 40%.

BMI was expected to take advantage of open-skies immediately by launching services in its own right. But its acquisition of another carrier, British Mediterranean, has led to a delay, with BMI not now expected to fly to America until 2009.

Its Heathrow slot pool has made it the subject of constant takeover speculation, with BA and Virgin constantly tipped as possible suitors. Virgin did hold exploratory talks with BMI five years ago, but they proved abortive.

Industry sources say, however, that BMI’s future is likely to be shaped by an agreement between Lufthansa, which owns a 49% stake, and Bishop, who owns the remainder. Lufthansa may choose to exercise its option to buy out Bishop next year – although it would have to reach agreement on price.

Lufthansa might then, one theory goes, look to consolidate Star’s position at Heathrow by going on to take a significant stake in Sir Richard Branson’s Virgin Atlantic. Singapore Airlines, another Star member, holds a 49% stake in Virgin but is understood to be willing to sell.

The battle for Heathrow has ramifications beyond its key North Atlantic routes. The open-skies agreement is expected to lead to a wider deregulation of the airline industry in Europe and America, and – eventually – to the scrapping of the ownership restrictions that have prevented mergers and acquisitions.

The winners at Heathrow will be strongly placed to consolidate their position once the restrictions are scrapped, and become the dominant players on the world stage.

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