HM Revenue and Customs (HMRC) has put back the tax deadline from January 31st following strike action planned by workers.
Members of the Public and Commercial Services (PCS) union are planning to take industrial action on the tax office's busiest day following a row over private firms running government-controlled call centres.
It means that any forms returned online in the first two days of February will not incur any fines.
Tax office workers reacted furiously when the HMRC announced that it was planning to trial run two companies, Sitel and Teleperformance, to operate the call centres in Cumbria and Bathgate, Scotland.
David Gauke, exchequer secretary to the Treasury, said: "This strike could have caused thousands of people to incur fines, so I am pleased that HMRC has taken this commonsense approach. The government does not want anyone trying to file their tax return on time to be unfairly penalised because they were unable to get through for help and advice on the 31st."