Staff will be balloted over industrial action after technology firm Capgemini warned of up to 600 jobs cuts.
The firm provides business and IT services to Her Majesty's Revenue and Customs (HMRC), which is currently carrying out a spending review.
Up to 400 jobs are under threat in Telford, Shropshire, about 200 in Basingstoke, Hampshire, and 100 in Worthing, West Sussex.
Staff voted "overwhelmingly" for the ballot, a union spokesman said.
Chris Morrison, from the Public and Commercial Services Union (PCSU), told BBC News that the union was working on the terms of the ballot before putting it before members in the New Year.
"In the past three weeks we have held meetings in Basingstoke, Worthing and Telford - the three main sites - and in each of those members have passed overwhelming support for the union to proceed to an industrial action ballot.
"We just need to go away and decide the terms and wording of the ballot and then we shall be in a position to ballot members in the New Year."
Discussions with Capgemini were on-going, he added.
The potential cuts could affect up to 20% of the 3,000 workforce around the country and staff will be told more details in March 2008.
Capgemini employs 2,200 people in Telford and is one of the largest employers there.
In 2005, the company said it wanted to hire 700 people across the West Midlands to work on its contract with HMRC.