Over 300 people employed by Fujitsu who are working in HMRC offices will go on strike after being given a pay rise lower than their civil service colleagues.
The workers are outsourced by Fujitsu Services UK and do a similar job to workers employed by the civil service.
The move could hinder the processing of tax returns.
According to the Public and Commercial Services Union (PCS), workers were offered a pay rise of 1.5 per cent compared to a five per cent pay rise offered to colleagues doing similar jobs.
Workers in Telford and other offices around the UK plan to strike on 30 January and 31 January, two days before the self-assessment online tax return deadline.
Around 87 per cent of PCS members voted for strike action.
PCS general secretary Fran Heathcote said that there is “no excuse” for workers employed by Fujitsu being offered less than those employed directly by HMRC.
“If the government was serious about its pre-election pledge to bring in the ‘biggest wave of insourcing in a generation’ now is the chance to end the scandal of a two-tier workforce,” she added. “It’s not too late for ministers to step in, resolve the pay issue and prevent strike action likely to play havoc with people’s tax returns.”