The deadline for filing tax returns has been delayed as thousands of tax office workers stage a 24-hour strike in a row over privatisation.
Members of the Public and Commercial Services union (PCS) across the UK will mount picket lines to protest at plans to trial the use of private firms at two centres.
HM Revenue and Customs has decided not to issue penalty fines to anyone missing Tuesday's deadline for self-assessment returns. Anyone filing their returns on February 1 or 2 will also not be fined.
The industrial action is in opposition to the appointment of two private companies, Sitel and Teleperformance, to run call-handling trials in HMRC tax credit contact centres in Lillyhall in Cumbria and Bathgate in Scotland.
The year-long trials are due to start next month and the union is warning they risk paving the way for privatisation in the department, and come at a time when tens of thousands of civil service jobs are being cut.
PCS general secretary Mark Serwotka said: "Our members in tax offices want to do a good job and provide the best possible advice and help to taxpayers, but there are fewer of them working in fewer offices as a result of misguided and damaging cuts.
"Instead of making even more cuts and throwing public money at private companies, ministers should be investing in their staff and tackling the billions in tax avoided and evaded by the super-rich."
An HMRC spokesman said: "HMRC is not privatising existing HMRC contact centre jobs but we are determined to improve the service we provide to our customers. This means considering a variety of options including drawing on the knowledge and experience of external contact centre operators. Industrial action is unwarranted and unnecessary.
"We are doing everything possible to maintain contact centre services to the public and will continue talking to the unions to address their concerns."
HMRC's acting director general for personal tax, Stephen Banyard, said: "We've always been very clear that we want the returns - not the penalties. For that reason, we don't want anyone who can't get through for help and advice on 31 January to be disadvantaged in any way."
© 2012 Press Associationby Labour. He indicated the Government would look at those arrangements although it had no plans to change them.