Southeastern train staff have begun voting on whether to strike over changes to jobs at stations in London.
The Rail, Maritime and Transport (RMT) union vote is over plans to reduce the number of team leaders at Charing Cross and Victoria stations from 14 to eight.
The union said the loss of team leaders would create "confusion and chaos" for passengers and compromise safety.
Southeastern said no jobs would be lost but the role of the team leader will change.
There are seven team leaders at each of the stations. They liaise between station management and staff members and also interact with customers.
Southeastern has reduced that figure to four at each station, but said the loss of six posts will not result in job cuts and those employees will be reassigned.
The company's spokesman said: "The changes at Victoria and Charing Cross have been in place since Sunday 5 December.
"These were already implemented months ago at Cannon Street and London Bridge following consultation with the RMT at these stations. The changes have worked well and have not compromised the safe operation of trains."
The RMT's general secretary Bob Crow said: "Southeastern have been caught out yet again cutting staffing and operational costs to maximise profits in a move that will not only cause chaos on the platforms at Victoria and Charing Cross but which would also compromise safety.
"RMT will not accept such penny-pinching behaviour and we are campaigning for a massive yes vote for action in this ballot."
The ballot closes on 4 January 2011.