Services provider Fujitsu has been warned that it could face possible financial penalties should it fail to meet agreed SLAs as a result of industrial action by members of PCS and Unite, which is set for 19 September.
Over 1,000 IT workers in government departments will strike for 24 hours later this month after the unions joined forces in protest at a pay offer.
The joint action will involve 720 PCS members in various locations and 307 Unite members based in Manchester, all of whom work on contracts in government departments including the DVLA and HMRC.
Fujitsu claimed the government departments will be unaffected by the 24 hour walk out. "We have robust contingency plans in place to ensure this," it said.
The company added: "Fujitsu is disappointed that PCS and Unite has announced industrial action on 19 September. We are continuing to work with both unions to resolve the disputes."
The unions said that 65% of PCS members and 85% of Unite members in Manchester voted for some form of industrial action, with 56% voting to strike.
"Such a pitiful pay offer to workers, who are paid less than what some senior executives pocket in bonuses alone, is an insult," said PCS general secretary Mark Serwotka.
"Our members may work for Fujitsu but they are supporting essential public services, so ministers must recognise they have a responsibility for what happens on behalf of their departments," he said.
Len McCluskey, Unite general secretary, urged Fujitsu management to meet union officials to try to negotiate a settlement.
Unite members in Crewe are also meeting to decide whether they should also take part in the joint action.
Reporting by Karl Flinders. A version of this story appears on ComputerWeekly.com