After 20 years of planning and protest, including a four-year public inquiry, and at a cost of £4.3 billion, Terminal 5 at Heathrow was formally opened yesterday by the Queen, ready for the first passenger departures on March 27. With soaring glass windows, spacious lounges, banks of check-in desks and a capacity to handle 30 million passengers a year, it will make an immediate difference to the beleaguered airport. In the past 18 months it has operated at breaking point: the queues, cramped conditions, frequent disruptions and improvised tents during security scares have earned Heathrow an unenviable and damaging reputation as one of the world's worst airports.

At least passenger spirits may now improve. The new terminal will be used by British Airways, which operates 40 per cent of all flights from Heathrow, allowing other airlines to move into Terminals 1 and 4 and making the rebuilding of “Heathrow East” less disruptive. But it would be naive to assume that the airport's troubles are over. Heathrow was originally designed for 45 million passengers a year, is now contending with 68 million and, if growth patterns continue, this total could rise to 122 million by 2030.

Many passengers now want BAA, the airport's owners, to concentrate on the improvements essential to make the airport function better as it is, rather than planning more retail outlets or getting bogged down in the financing of future expansion. But a new terminal can go only so far in relieving congestion. The main challenge for the airport is in the sky: at peak times so many planes are waiting to land that many have to be held in stacks, wasting time and fuel, adding to pollution and passenger frustration. What needs soon to be decided is whether to end the current system of the alternate use of runways, which switches planes taking off on one runway to the other halfway through the day to give nearby residents respite. By allowing take-offs and landings on the same runway, extra departures can be slotted in during lulls in arrivals.

This will be controversial not only because of the increased noise but because it will be seen as a stepping stone towards the construction of a third runway and a sixth terminal, a development that the Government supports. The only way it could mollify opposition would be to maintain the yearly cap of 480,000 flights. That limit has almost been reached - there are now some 472,000 flights a year. But if expansion is, for now, put on hold, there must be provision for London elsewhere. The obvious place is Gatwick.

Heathrow, however, is what matters overwhelmingly to London's international competitiveness. And the doubts over the capacity of Ferrovial, BAA's Spanish owner, to manage this premier airport together with Gatwick and Stan- sted are growing. The huge burden of debt is already crippling all investment plans. Airlines and passengers have been angered by the extra landing charges that BAA has been permitted to levy, believing they should not be obliged to get Ferrovial out of its financial hole. The Commons Transport Select Committee says today that breaking BAA's monopoly is the only way to encourage competition and efficiency and improve Heathrow. It is right. And unless Heathrow shows rapid and permanent improvement in the quality of its service to its passengers, BAA may find that this is the airport it is obliged to sell.