By Richard Scott
BBC transport correspondent
Pilots could get an extra £600 a year if their union wins the case
British Airways is due in court to fight a union's attempt to change its policy on holiday pay for pilots.
Pilots' union Balpa has gone to the Supreme Court to argue that total pay - including allowances - must be used in the calculation, not just basic salary.
Loss-making BA is resisting the claim, which could result in 15,000 staff each getting an average of £600 a year more.
The airline is already battling a separate strike threat from cabin crew over changes to working conditions.
Pilots receive a basic salary, as well as allowances for flying time, night flying and time away from base.
British Airways uses the basic salary to calculate pilots' holiday pay. The union argues it should be based on total pay, including allowances.
The pilots' union Balpa, says that Working Time Regulations set out how holiday pay should be calculated - as an average of the last 12 weeks' pay.
But those regulations do not apply to the airline industry, which is covered by the Civil Aviation Working Time Regulations. The union argues these don't specify how holiday pay should be calculated.
And it is not just BA which will be affected by the union's stance.
"We have tribunals lodged against Virgin, BMI, Easyjet and Cityflyer," said Balpa general secretary Jim McAuslan.
"We are also making the case that in the UK, the civil aviation regulations should be amended to give workers in our industry the same right to proper holiday pay that everyone else gets."
Four years ago the union won an employment tribunal on the issue, but the case eventually moved to the Court of Appeal which backed BA's side of the argument.
Now the new Supreme Court is being asked for its ruling.
"We have always been of the view that our holiday pay arrangements are generous and comply with legislative requirements," said a BA spokesperson. "We will continue to resist the claim strongly."
The case will also affect cabin crew holiday pay. In total about 3,000 BA pilots and 12,000 cabin crew could each get an average of £600 a year extra. That would cost BA about £9m.
It adds to the problems facing the airline, which is embroiled in a long-running dispute with its cabin crews over changes to the way they work.
This week members of the Unite union voted overwhelmingly to go on strike over the issue, though no dates have yet been set.