The dispute is over all staff getting enhanced Sunday pay
Rail workers on London Midland routes are to be balloted over a campaign of industrial action in a row over pay.
The Rail, Maritime and Transport union has announced it is to hold the ballot.
A spokesman said station, clerical, retail, booking office and supervisory staff did not receive the same pay for Sundays as drivers.
London Midland services operate between London, the north of England, the Midlands, Northamptonshire, Bedfordshire and Buckinghamshire.
General Secretary Bob Crow said feelings were running high among staff who were not getting the same enhanced Sunday pay levels as drivers.
He said: "The RMT believe that all grades should be treated equally."
The ballot will begin next Tuesday and end on 30 July.
London Midland are expected to comment later.