British Airways has delayed moving most of its long-haul flights to Heathrow's new Terminal 5 until June.
"It is only sensible to ensure that Terminal 5 is operating consistently at a high standard before the move begins," said BA boss Willie Walsh.
The new £4.3bn terminal has been plagued by problems, especially with its baggage handling, since it opened.
Almost all of BA's long-haul flights were supposed to switch from Terminal 4 to Terminal 5 on 30 April.
BA boss defends Terminal 5 postponement
No date in June has yet been given for the move.
The delay in moving will have a knock-on effect on other airlines, which had been due to move into Terminal 4 once BA had left.
Those due to move into Terminal 4 include Air France-KLM, Delta and Continental Airlines.
BMI said the delay was "outrageous and inconsiderate", adding that the UK "was once again a laughing stock".
In a joint statement with airport operator BAA, British Airways said: "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's Mr Walsh added that he thought it was a sensible decision.
"We've clearly learned from our experiences of working in Terminal 5 since we've moved in there and it's a decision taken with our customers in mind," he told the BBC.
BAA regrets this postponement and we recognise the impact it has on other airlines
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