Staff at Steria, working on an account serving Capita, will go on strike tomorrow over a pay dispute.
The staff, represented by the Unite trade union, voted by a large majority - some 82 percent - to take the action, in what has become an increasingly bitter argument.
Unite said the staff were refused a 2010 pay rise, which would have been paid in April this year.
Tomorrow's strike will involve system developers and analysts, and they will picket at Steria sites in Bristol, Coventry and High Wycombe. The strike will operate for 24 hours, from midnight tonight.
"This is the first time that this group of workers have ever taken these steps as they feel that they have no option other than to withdraw their labour in protest at the refusal of Steria to make any kind off pay offer whatsoever," said Unite regional officer, Jerry Pickford.
"Over the last few years, our members have seen their workload increase tremendously and for nearly a decade they have received below inflation pay rises, which in real terms is seen as a pay cut. They have now reached the point where they are saying 'enough is enough'."
Pickford said Steria cited the economic climate as a reason not to award a pay rise, but added that the company "continues to post excellent results".
The IT services company said in a statement: "Steria has carried out a comprehensive pay review awarding pay rises to our lower paid employees this year. We cannot add any further costs to our pay bill. At this time of increasing economic uncertainty our priority is to protect the job security of our employees.''