Some 55,000 Revenue and Customs (HMRC) staff are going on strike over job cuts and "creeping privatisation".
Public and Commercial Services union members are protesting about plans to cut 10,000 HMRC posts by 2014/2015.
The union said a lack of resources meant billions in tax was lost annually through evasion and avoidance and the "massive cuts" would worsen the issue.
HMRC said it was on target to increase its tax take from compliance work and was "disappointed" by the strike.
"We are seeking dialogue with the PCS to address their concerns and will work to minimise any disruption to our customers," a spokesman said.
"In our 2010 spending review the government made £917m available to us to tackle avoidance, evasion and fraud. This is being used to increase our tax take from compliance work by £7bn a year in 2014/15 which we are on target to do."
'Off the hook'The PCS said the latest plans for job cuts follow about 30,000 job losses since the department was formed in 2005.
In May, the Public Accounts Committee said an additional £1.1bn in potential tax revenue could have been collected over the previous five years if compliance and enforcement staffing levels had not been cut.
General secretary Mark Serwotka said the case for investment in public services "could not be more obvious than it is at the HMRC".
"Yet the government wants to cut 10,000 more jobs from the department, letting the wealthy tax dodgers off the hook and punishing the rest of us for a recession we did not cause," he said.
The union says it is also opposed to year-long trials using private companies to handle tax credit enquiries at two centres in Lillyhall, Cumbria, and Bathgate, West Lothian.
Mr Serwotka has described it as "creeping privatisation" in HMRC.
The strike will be followed by other forms of action such as a ban on overtime.
PCS members were among tens of thousands of public sector workers who joined a 24-hour UK-wide strike on 10 May over government changes to pensions.
