Starting next week, thousands of volunteers will be spend a day at Heathrow's new Terminal 5. In return for a free lunch and a "goody-bag", they will help test the systems at the £4.3bn project. But far from solving the airport's problems overnight, the controversial new terminal will precipitate a summer of confusion at Heathrow - and seriously diminish public transport links for millions of the users of Europe's busiest airport.

The front page of the latest edition of British Airways News is headlined "T5: Our amazing new home". Last week, the airline took possession of the new building, to begin testing the systems in advance of its scheduled opening on 27 March next year.

"Terminal 5 is a fantastic opportunity for British Airways and will transform our operations," said the airline's chief executive, Willie Walsh. But for many frequent travellers the new building will only be the start of disruption.

Originally, the new terminal was intended to house BA's entire Heathrow operation. But the building is already too small. Next year, BA ticket holders will be using not just Terminal 5 but Terminals 1, 2, 3 and 4 as well. And even when the transition phases are finished, travellers to key destinations – including Barcelona, Nice, Singapore and Sydney – will use a different terminal with awkward connections that could add a couple of hours to a journey.

When Terminal 5 opens, BA's long-haul flights from Terminals 1 and 3 will move in – together with most short-haul flights from Terminal 1 and 4. But during the five weeks before long-haul services from Terminal 4 move across, connecting passengers will have to endure even longer transfers than they do at present. By the end of April, most flights will be "co-sited" at Terminal 5 – but BA flights to Singapore, Sydney and Bangkok will remain in Terminal 4 all summer, then shift to Terminal 3.

Frequent travellers on short-haul flights also face multiple complications. Services to Barcelona, Madrid, Belgrade, Lisbon, Nice, and Helsinki will remain at Terminal 1 to begin with, along with flights to the Mediterranean and North Africa operated by GB Airways (on behalf of BA). These will shift to Terminal 3.

"The roadside signs explaining which terminal to go to will need to begin at Hyde Park Corner," said an aviation source. "It's going to be unbelievably complicated."

BAA, which owns the airport, has a policy of persuading travellers to arrive by public transport. But the implications of the new terminal for Tube and Heathrow Express services are worrying. Terminal 5 will be served by the Heathrow Express, but only at the expense of users of Terminal 4. At peak times Terminal 4 users will have to change trains at the airport's central station or use the slower Heathrow Connect service.

The service on the Picadilly Tube line from central London will also be split; all trains will serve the central area, but services to Terminal 4 and Terminal 5 will run at 10-minute intervals. BA's move to Terminal 5 will precipitate a long, complex sequence of moves by other airlines. The eventual plan is for the leading alliances – Star, Oneworld and Skyteam – to be co-sited in Terminals 1, 3 and 4. "Fundamentally this is the key that unlocks the transformation of Heathrow, to the benefit of passengers and airlines," said Mike Forster, director of strategy and development, the BAA executive who is choreographing the move. But there is no denying the fact that moving 54 of the 88 airlines currently operating out of Heathrow is potentially a logistical nightmare.

By numbers

68 million: Number of passengers going through Heathrow per year

30 million: Number of passengers expected to pass through Terminal 5

202,280: Number of BA flights per year

475,000: Number of flights overall per annum

480,000: Maximum allowed: