Staff at Fujitsu, the technology firm at the centre of the Post Office Horizon scandal, are due to go on strike tomorrow, which their trade union said could disrupt HM Revenue and Customs at the busiest time of year for tax collection. About 300 staff, most of whom work in IT support for HMRC at sites in Telford and Stratford, east London, will go on strike in protest at a pay offer that the Public and Commercial Services union (PCS) said was 10 times less than what Fujitsu is offering staff in Japan. The PCS said Fujitsu staff involved in fixing and protecting HMRC computers would take further action between 18 January and 15 February. HMRC is understood to be confident that industrial action would not affect its ability to process self-assessment tax returns in the busy weeks before the 31 January deadline. However, the PCS said it believed the strike was “likely to disrupt those seeking to complete their self-assessment tax returns by the end of the month”. The union said the strike had nothing to do with the faulty Horizon IT system, which produced incorrect data that the Post Office used to wrongly accuse thousands of postmasters of theft and fraud. That scandal is expected to cost taxpayers at least £1bn in compensation and Fujitsu said on Tuesday it bore moral responsibility to contribute financially. The company has defended its continued role in government contracts, including for the Ministry of Defence and HMRC. The PCS general secretary, Mark Serwotka, said: “Why is the UK government giving Fujitsu lucrative contracts when the company treats its UK workers so badly? “Why is it workers in Japan are deemed worthy of a pay rise almost 10 times more than those in the UK? “Fujitsu made £22m profit last year in the UK, so it can afford to pay our members a decent wage – it just chooses not to. Fujitsu said: “We remain committed to continue pay talks to try to resolve this situation and welcome further discussions at any time. “Fujitsu will be taking all necessary steps to ensure operational continuity for all our customers and are working closely with those customers throughout the planning process.” An HMRC spokesperson said: “We have robust plans in place to ensure we continue delivering critical services for our customers during any industrial action.”