Virgin Atlantic is a key transatlantic carrier operating from main airports

In a letter to 4,800 cabin crew, the Virgin boss warned he would not be meeting pay demands.

Workers' union Unite called the letter from Sir Richard "provocative".

It comes after union members voted to strike in January in protest over pay levels they say are lower than at other airlines, including British Airways.

The 48-hour strikes are scheduled for 9 and 10 January, and 16 and 17 January.

For some of you, more pay than Virgin Atlantic can afford may be critical to your lifestyle and if that is the case you should consider working elsewhere

Sir Richard Branson

The strikes are not the only industrial action scheduled by aviation workers for January.

Union members at airport operator BAA had called for strikes on 7, 14 and 17-18 January.

However, the first of the three strikes - set for 7 January - was called off on Monday.

'Dangerous precedent'?

In his letter, Sir Richard said Virgin Atlantic had "drawn a line in the sand" over pay this year.

"To go further would result in unacceptable risks and would set a dangerous precedent to the company as a whole," he added.

"It would be irresponsible of our management and they, rightly, are not going to take that risk."

Sir Richard admitted that rival airlines often offered better basic wages but said that they did not offer the perks that came with working for a "smaller, more friendly" company.

And he added: "For some of you, more pay than Virgin Atlantic can afford may be critical to your lifestyle and if that is the case you should consider working elsewhere."

'Unhelpful'

Virgin Atlantic has previously said that there would be no "eleventh hour change to our pay offer".

The airline said its offer was worth 8.3% on basic pay over two years, with a 4.8% increase offered in the first year.

A Unite spokesperson said the letter was "unhelpful in resolving the dispute" and would "only make people more upset".

A Virgin Atlantic spokesperson said that since the letter was sent, a number of union members had contacted management offering to cross the picket line and work on the strike days.