British Airways is to postpone moving most of its long-haul flights to Heathrow's T5 because of continuing problems at the £4.3 billion terminal, union sources said.
The airline was due to transfer nearly all its long-haul operation from Heathrow's Terminal 4 on April 30.
But the opening of the new terminal has been beset by delays and disruption, with flights cancelled and passengers left stranded.
Paul Kenny, general secretary of the GMB union said: "This is a sensible move which gives time to make sure everything works smoothly."
BA and airports operator BAA later announced that the planned move of the airline's long haul services will be deferred until June. They said in a joint statement: "While a normal flight schedule has been operated at Terminal 5 since Tuesday, we want to ensure that customers can be confident of high service standards when additional flights are introduced.
"We will work together to iron out any remaining problems, including the baggage system and its operation, and develop a robust timescale for phasing the move of Terminal 4 services into Terminal 5."
BA chief executive Willie Walsh said: "We are making this decision in the interests of customers. Though Terminal 5 is now working well, we need to have confidence that good service can be maintained when the terminal is handling larger numbers of customers.
"It is only sensible to ensure that Terminal 5 is operating consistently at a high standard before the move begins."
BAA chief executive Colin Matthews said: "BAA regrets this postponement, and we recognise the impact it has on other airlines, but we believe it is a wise precaution to ensure that passengers can have the maximum confidence once the move does take place."
Under the original plans, almost all BA's Terminal 4 long-haul services were due to switch to T5 at the end of the month, totalling 60 departures and 60 arrivals a day.
