British Airways has launched an official investigation into the wildcat strikes by 1,000 of its ground staff last week that brought Heathrow Airport to a standstill and could have cost it up to £50 million.

The airline, which was forced to cancel 700 flights on Friday and over the weekend, will try to establish as part of the investigation whether staff "were bullied or intimidated into walking out or staying put".

BA has set up a hotline for staff following reports that ground crews might have been forced into taking the unofficial action. The airline said this morning that it had yet to receive a response from the Transport and General Workers Union over whether it would co-operate with the investigation.

BA described the investigation as a "fact-finding exercise" and would not be drawn on what action it would take if it discovered evidence of bully-boy tactics by shop stewards or other staff members.

It said simply that it would be taking "appropriate action".

The wildcat strikes, which caused delays for 100,000 passengers trying to fly in and out of Heathrow, were staged by BA baggage handlers, cargo workers and bus drivers, in support of 670 sacked staff at caterer Gate Gourmet.

BA said this morning that Gate Gourmet was not included in its investigation.

Sir Rod Eddington, BA's chief executive, described the strikes as "outrageous" and has apologised to passengers. The airline will be forced to bear the costs of having re-booked, re-routed or refunded the delayed fliers.

In an open letter to staff, Sir Rod said the strikes were "a body blow that defies belief". He said: "Never before have we had to cancel the entire operation at our worldwide base.

"Those who took unofficial industrial action at British Airways acted in an unlawful way. As a result, they will not be paid for the time they were unlawfully absent from work.

"I have also launched a full investigation into the circumstances that brought about this unofficial and outrageous stoppage so that further appropriate action can be taken."

He said the stoppage had put the airline under considerable and unnecessary pressure.

BA insisted that the phone line will be operated in complete confidence and callers' names will only be disclosed with the caller's consent.

Although analysts have already estimated that the unofficial walkouts will cost BA as much as £50 million, the airline said it was still too early to say how long it would take to completely restore service levels or what the long-term implications of the industrial action might be.

Martin George, commercial director of British Airways, wrote to regular fliers yesterday, with an update on the inflight catering.

He wrote: "Although British Airways services into and out of London Heathrow continue to operate as normal, we have limited onboard catering.

"There will be some catering provided at the airport prior to departure, however we are continuing to advise passengers, especially those with special dietary requirements, to eat before arriving at the airport."

Gate Gourmet yesterday brought in a team of forensic accountants to investigate claims by the TGWU that it had been involved in "improper corporate activities" in its dealings with BA. The allegations were levelled by union leader Tony Woodley at a meeting with the caterer yesterday.

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