VIRGIN Blue could be forced to pay millions of dollars in back pay to ground staff who failed to receive shift allowances.

The airline accidentally left a shift allowance out of the pay packets of its ground handlers and guest services staff from August 2000 until October 2004.

The Australian Services Union has lodged a claim to have the matter heard in the Australian Industrial Relations Commission in Melbourne on Tuesday.

While ground staff worked an average of eight hours a day, Virgin's (vba.ASX:Quote,News) pay office miscalculated that staff had worked 7.5 hours, and paid shift allowances accordingly.

The episode had cast a cloud over hopes Virgin Blue had overcome its recent profit slump.

Virgin has declined to comment.