Planned strikes by BA cabin crew today handed a pre-Christmas bonus to rivals benefiting from the "ABBA" – Anything But BA – effect as industry experts predicted a "slow attrition" of the airline's customer base.

Virgin Atlantic said many thousands of passengers had already defected from BA due to the uncertainty. "This was rumoured back in the summer, and clearly there will now be many more trying to switch across," said Paul Charles, Virgin's director of communications.

"No doubt, it's going to have a huge impact. BA's phones are going to be ringing red hot. If people's holidays are ruined over Christmas and New Year, they will think twice about who they fly with in future."

Virgin would do "everything we can" to accommodate those affected, "but this is one of the busiest times of the year and many of our flights are already full".

RyanAir said it was set for a record number of passengers this Christmas. "A lot of that growth can be attributed to the fact people weren't taking chances with BA," said spokesman Stephen McNamara.

Flybe and easyJet were also poised to take advantage. Flybe pledged to "look to increase our services where necessary to assist BA and their passengers to make sure everyone gets to travel during the festive season and beyond".

The big question was how much the strike would hit BA financially, said Murdo Morrison, editor of Flight International magazine. "They are in a reasonably cash rich position, so the strike itself won't have an immediate effect financially, but it will have on their whole image as a carrier, particularly competing with low-cost airlines," he said.

"It used to be BA was the default airline for business traffic into and out of the UK. Now, as BA have cut their frills, ironically, and the no-frills airlines have increased their networks, and the whole stigma thing has gone from using a low-cost airline, a lot more business people are using the RyanAirs and easyJets."

He added: "I don't think there will be a seismic change in passenger behaviour just from this strike alone, because people will return to BA. But probably fewer will return next time. So, they will lose passengers, certainly in the short term due to the strike, and also there will be slow attrition of their customer base in the long term."

Passengers were advised that BA was obliged to offer a refund for cancelled flights or re-route passengers to other airlines. "But for many, particularly at Christmas when it is very busy, that might not be of much use," said James Freemantle, of the Air Transport Users Council.

BA was also obliged to offer assistance, where necessary, for hotels and meals. What was less clear was whether BA was obliged to pay compensation. Regulations covering cancellations state that airlines should pay unless there were certain circumstances.

"One of those circumstances is if there is a strike that affects the operation of the aircraft," he said. "But there could be the opinion that refers to strikes outside the airlines' control. There has been no case law. In simple terms, passengers may want to take up the issue of compensation with BA, but it may be they have to go to court to do it."

Frances Tuke, a spokeswoman for the Association of British Travel Agents, described the strike announcement as bad news for the travel industry.

"People who have booked Christmas and new year package holidays involving BA flights will find that tour operators will either have to re-arrange flights for them or give them a refund," she said. "However, this is an extremely busy time of the year for air travel and it may be very difficult to find alternative flights."

Passengers should "sit tight" for the next 24 hours. "It looks as if both sides are continuing to negotiate and we're still hoping it might be called off."

Travel firms were awaiting confirmation from BA about which flights would be affected, she added. "We are waiting for a schedule from BA because there are staff that are not unionised and there is a potential pool of workers that could be used to operate flights."

Bob Atkinson of travelsupermarket.com said: "Customers who booked before the ballot announcement on 2 November and have insurance to cover for strike action will be able to recover losses from making new bookings. However, customers who booked after 2 November, or those with no insurance, will be left high and dry to find new arrangements. They will be refunded for the price of their original ticket by BA and even if BA try to find alternatives for customers availability is already very tight."

People who booked direct with BA were being advised to contact the airline for a refund or a change to their flight time. Package holidaymakers were being told to contact tour operators to ask for their money back or to book different flights.