Workers at HMRC are to strike across the UK next week in protest over jobs cuts and the closure of walk-in tax-enquiry centres.
A spokesman for the Public and Commercial Services union, which has about 60,000 members at HMRC, said there will be daily strikes next week. Regions will strike on different days.
HMRC plans to cut about 10,000 jobs by 2015. It has cut about 37,000 jobs since 2005, according to the PCS.
The PCS says that a lack of resources means that billions of pounds in tax is lost each year through evasion and avoidance and cuts will worsen things.
"The dispute is about staffing," a PCS spokesman said. "Years of successive job cuts have left HMRC unable to cope, with delays on telephone lines, backlogs of post and private debt collectors brought in to chase up tax credits over-payments."
The union also said that it opposes plans to "privatise more of [HMRC's] debt collection and mail handling".
HMRC has said it has increased resources to tackle tax evasion and is improving training.
There will be five days of strikes next week.
HMRC workers in Scotland and north-east England will strike on Monday, Yorkshire and Humberside, and eastern England (Tuesday), London, the south east, and south-west England (Wednesday), Midlands and Northern Ireland (Thursday) and north-west England and Wales (Friday).
Industrial relations at HMRC have been strained since the merger in 2005 between the Inland Revenue and Customs and Excise. Waves of job cuts, changes to pensions and work practices have sparked regular strikes.
In February, senior tax officials went on strike on Valentine's Day in protest over a new employee appraisal system, which tells 10% of workers that they are under-performing and need to improve.