Virgin Trains has delayed the introduction of new uniforms after some female staff said blouses were see through and too low cut.
The rail operator is now offering £20 vouchers for women employees to buy undergarments to save them any embarrassment.
The problem came to light after trials of a new uniform for staff on Virgin's West Coast Main Line.
Virgin said it consulted widely with staff but it was not clear how the new uniform would be received until it was actually worn.
The Transport Salaried Staffs Association said staff had complained that the red-coloured blouses were too flimsy and would allow male passengers to see dark bras being worn underneath.
General secretary Manuel Cortes said: "Our female members are upset because they feel Sir Richard Branson is cutting corners by asking them to wear flimsy blouses which are skimpy and they feel too revealing.
"He has asked Vivienne Westwood to design the new uniform for his air crews. Unfortunately, by contrast, it looks like he is getting the blouses for train crews from Del Boy at Trotters Independent Traders Ltd at Peckham Market."
A Virgin Trains spokesman said the launch of the new uniform had been delayed from this week to the end of the month to allow more feedback from staff.