People who miss next week's tax self-assessment deadline will not be fined because of a strike by tax-office staff.

HM Revenue and Customs (HMRC) said it will not be issuing penalties to anyone who files their tax return on February 1 or 2. Equally, those who have tax to pay will not face any interest on payments made on February 1 or 2.

HMRC's acting director general of 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 January 31 to be disadvantaged in any way."

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 common-sense 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."

Up to 20,000 members of the Public and Commercial Services union (PCS) working in call centres and inquiry offices took strike action last week in a row over the appointment of private companies to run call-handling trials in two contact centres, and will walk out for 24 hours next Tuesday.

A union spokesman said: "If the minister genuinely had taxpayers' interests at heart, he would call off the trials and invest in his own staff and the essential services they provide, instead of making cuts and letting in the private sector.

"That way, he might also be able to show a commitment to seriously tackling the uncollected billions of pounds of tax - not generally from ordinary taxpayers, but largely from very wealthy individuals and organisations that avoid paying what they owe."

The action is in protest at 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.