HM Revenue and Customs workers struck on 31 January.
The strike was in opposition to the appointment of private companies (Sitel and Teleperformance) to run call-handling trials in HMRC contact centres in Cumbria and Bathgate (Scotland).
The trials are due to begin in February, and the Public and Commercial Services union (PCS) warns that they could open to door to further privatisation within HMRC.
The strike was timed to coincide with the deadline for self-assessment tax returns, the busiest day in HMRC’s calendar, and has led to an effective two-day extension of the deadline by HMRC bosses.
Only PCS members who work in Personal Tax Operations (PTOps) have been balloted to take part in the action.
However, the union is encouraging other HMRC members to support the strike by donating to the strike fund and refusing to cover work not being done by striking members.