More than 1 million British Airways passengers have been spared the Terminal 5 experience after the airline postponed the full transfer to its new Heathrow home by at least a month.

BA admitted it cannot cope with a normal schedule at T5 as it yesterday announced a delay in transferring its long-haul services from April 30 until June at the earliest. The move caught out thousands of passengers booked on connecting flights to T5 over the next month, some of whom could be in danger of missing their onward trips because they will have to pass through Terminal 4 instead.

The BA chief executive, Willie Walsh, admitted that sticking to the original deadline would present an "unnecessary risk" after the blighted T5 opening, which led to more than 500 cancelled flights due to a chaotic baggage handling operation.

The airline and Heathrow owner, BAA, are still struggling with glitches in the luggage system and ongoing problems with the £4.3bn building. BA is supposed to handle more than 2 million passengers a month at T5, but for now around half its passengers will stay at T4.

"If we were confident, we would move forward, but we are taking it in small steps," said Walsh. "I am conscious of the fact that a lot of people have said we should have taken smaller steps, so you should be welcoming this as a sensible decision on the part of BA and BAA."

BA staff are scrutinising reservations for connecting services at T5 as they prepare to warn some passengers that they will miss their onward flights. The BA schedule assumed passengers would be flying via T5, but the transfer time will double if Terminal 4 is involved as customers are ferried across the airport. The transfer of long-haul services to the new terminal will double T5's capacity from its current 40,000 passengers a day to 80,000.

Walsh declined to give a definite date for the T4 move, saying a "phased" transfer would start on June 5 at the earliest. He denied BA had postponed the move over fears of a repeat of T5's opening day on March 27, when the baggage handling operation fell apart. "We clearly got a lot of things wrong on March 27 but we have moved on since then," he said, referring to four successive days of smooth operations at T5 this week.

So far, 693 out of 5,277 flights in and out of T5 have been cancelled, with 75% of the cancellations due to T5 problems and the rest down to extreme weather.

"We still have some issues that we need to address and we want to build on progress we have made more recently," Walsh said. "Not all of the facilities are working as we expected. Not all of the lifts are working."

He said outstanding "small issues" would become "more significant" if BA doubled passenger volumes too soon, adding that it was taking baggage handlers, some of whom were disoriented by the T5 layout, "a bit of time" to get used to the system.

Walsh also shrugged off a scathing letter from the pilots' union, Balpa, which blamed the debacle on senior management at the airline. "I don't lose any sleep or get concerned about issues like that."

Meanwhile, the T5 disruption spread to the rest of Heathrow and 54 other carriers following BA's announcement yesterday. Other airlines are expected to seek compensation.

The delay will disrupt plans by carriers such as Air France-KLM to move into a refurbished Terminal 4. BMI, which operates from Terminal 1, criticised the decision and said BAA had made the UK "once again a laughing stock".

Nigel Turner, BMI chief executive, said: "It is an absolutely outrageous announcement by BAA and done with no thought, consideration or consultation of any other airline other than BA. The sequence of moves affects over 50 airlines, including BMI, at Heathrow. The programme and timescale of changes was agreed in joint consultation with all airlines that are now geared up to undertake the moves as agreed."

FAQ Terminal phase

Why can't BA make the full switch?

The baggage handling operation is still suffering problems and cannot cope with a doubling of volumes. Instead, the extra pressure will be "phased" to prevent meltdown.

When will all the problems be fixed?

BA says T5 is running more smoothly, but it wants more "bedding in" time to fully acquaint baggage handlers with the system. BA will start adding services and passengers in June, but won't say when it will run at full capacity.

Will the delay affect all of Heathrow?

Air France-KLM, the world's largest airline, is due to move into T4 with Alitalia and Russia's Aeroflot, while Air New Zealand and United Airlines are due to switch to T1. BMI says the plans and bookings for millions of passengers have been thrown into chaos.