Britain's major airport will be even busier when Terminal 5 opens, but getting there from central London could be a nightmare. Simon Calder looks down the Tube

Seventeen years after it opened, the newest terminal at London's Heathrow airport is already woefully inadequate. Indeed, so overstretched is Terminal 4 that British Airways is starting to move some of its long-haul flights to the ancient Terminal 1 in order to relieve some of the pressure on its main intercontinental base.

Five years from now, the much-debated and delayed Terminal 5 is due to open at Britain's busiest airport. This should allow BA to concentrate its whole operation at a single terminal. But the consequences for other travellers using public transport to reach Heathrow will be considerable.

When Terminal 4 opened in 1986, it was sited a long way away from the rest of the complex, on the south side of the airport. New tunnels had to be dug to allow the London Underground network to reach the site. A curious loop arrangement means that Tube travellers to the main part of the airport find their journey stretched by five minutes because of the subterranean detour around the south of the airport. "The longest five minutes in travel is the time it takes from Heathrow Terminal 4 to Heathrow Central on the Piccadilly Line," says one jaded traveller. Five years ago, when the Heathrow Express rail link was finally opened, more tunnels had to be dug beneath the runway to connect the airport's central area with Terminal 4.

What will happen to both these links when Terminal 5 opens? Look closely at future maps of the London Underground system, because the western end of the Piccadilly Line is set take on an extremely odd appearance.

The good news is that London Underground plans to run 15 trains an hour to Heathrow, compared with the existing scheduled 12. Those lucky enough to be travelling to Terminals 1, 2 or 3 should find that there is one Piccadilly Line train every four minutes or so.

But those aiming for Terminal 4 or 5 will be less happy. A spur is to be built from Heathrow Central to Terminal 5, where trains will terminate. They will return to Heathrow Central and continue back into London. But which trains? Only the two out of three that London Underground plans to send to Terminal 5. The remaining third will be sent around the existing loop from Hatton Cross to Terminal 4, and back to Heathrow Central, where there is plenty of scope for congestion with the Terminal 5 trains.

What will worry travellers using Terminal 4, though, is the fear that Heathrow's new terminal could seriously delay their journey to the airport. The new arrangements will cut the number of trains per hour from 12 to five. Given the propensity for delays, especially on rush-hour services, and the need to take commuters on the other Piccadilly Line branch to Rayners Lane, passengers will need to allow much longer for their journeys.

Travellers heading for the central area on the Tube will be faced with a conundrum; if the first train is going the long way around via Terminal 4, should they take it or wait for one behind that will go straight to Heathrow Central?

All this is confusing enough for regular users of the Tube, but overseas arrivals making their first visit to London will find it baffling. The Tube's operator, meanwhile, says: "The design capacity will enable London Underground to increase services to match future demand when required."

The Heathrow Express, which is Britain's most expensive railway, has a simpler scheme: the line from London Paddington, which presently serves Heathrow Central and then Terminal 4, will be extended to Terminal 5. But journey times to the new terminal will be twice as long as the 15 minutes it now takes from central London to Terminals 1, 2 and 3. At present, the journey time to Terminal 4 is at least 23 minutes, and this is likely to extend to half an hour to Terminal 5.

There is some good news for people travelling between west London and Heathrow airport; an additional stopping service is planned to run from Paddington to Heathrow, stopping at Ealing Broadway, West Ealing, Hanwell, Southall, Hayes and Heathrow Central. But anyone hoping to reach Terminal 4 - and later, Terminal 5 - will then have to link up with the existing Heathrow Express service.

Additional research by Daniel McAllister