Sir Richard Branson has angered Virgin Atlantic staff who are threatening to strike by telling them they should “consider working elsewhere”.

The entrepreneur, estimated to be worth £3.1 billion, told 4,800 cabin crew in a letter that he would not meet their pay demands. He said: “For some of you, more pay than Virgin Atlantic can afford may be critical to your life-style and if that is the case you should consider working elsewhere.”

Union members have voted to stage 48-hour strikes on January 9-10 and January 16-17.

Sir Richard said he had drawn a line in the sand in the dispute with staff belonging to the Unite union, which is embroiled in separate negotiations with BAA, the airports operator.

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“To go further would result in unacceptable risks and would set a dangerous precedent to the company,” he wrote. “It would be irresponsible of our management and they, rightly, are not going to take that risk.”

A spokesman for Unite said the letter was “insulting” and would worsen relations with employees.

The carrier has offered 8.3 per cent on basic pay over two years, with a 4.8 per cent increase in the first year. Sir Richard argued that staff using British Airways wages as a benchmark were being unrealistic. “It’s like comparing chalk with cheese,” he wrote, adding that Virgin Atlantic did not have the benefit of a history of state support and priority landing slots.

Paul Charles, Virgin Atlantic’s director of communications, said: “We have been inundated with calls from staff saying ‘we want to come into work to keep the airline flying’.”