Tax workers are to stage a series of fresh strikes in a long-running dispute over job losses and office closures.
Members of the Public and Commercial Services union (PCS) will take part in walkouts over the next three days, threatening disruption to the work of HM Revenue and Customs (HMRC).
The union claimed that years of job cuts had led to backlogs and delays as well as the use of private debt collectors.
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The union is campaigning against the closure of enquiry centres and the loss of 2,000 jobs.
Strikes will be held across Wales and north west England tomorrow, Scotland and the Midlands on Thursday, and London, the south east, south west and east of England, Yorkshire and Humberside and Northern Ireland on Friday.
PCS general secretary Mark Serwotka said: "HMRC plays an essential role in our economy and our society, collecting the taxes that fund the other public services we all rely on. But it is being systematically undermined by unnecessary and politically-motivated cuts.
"These strikes demonstrate we are serious about stopping these damaging cuts and making a positive case for proper investment in this crucial department."
A HMRC spokesperson said: "We are very disappointed by the timing of the decision by PCS to call a strike to coincide with the tax credits renewals deadline.
"It is a great shame that the union is asking HMRC staff to strike, deliberately putting the livelihoods of hard working families at risk to further an industrial dispute.
"We will do everything we can to minimise the impact on tax credits claimants, and we urge them to renew online at www.gov.uk/renewtaxcredits, to avoid the long delays that the strike will cause on the phone."
The union said the strikes were timed to coincide with the deadline for tax credit renewals and a key date for self assessment payments.
The walkouts follow a series of rolling stoppages last month.