Tax offices were closed today and calls to telephone inquiry lines were met with a recorded announcement because of a strike by tens of thousands of Revenue and Customs (HMRC) workers.
Members of the Public and Commercial Services union (PCS) took action in protest at "massive" job cuts they claim undermined efforts to clamp down on tax avoidance schemes.
The union said 30,000 jobs have been axed since 2005 and a further 10,000 are set to go, "undermining" efforts to investigate schemes such as the one used by comedian Jimmy Carr, which sparked a political row last week.
The PCS said an estimated £120 billion was lost every year because of tax evasion and avoidance, claiming that HMRC did not have enough resources to tackle the problem.
The strike is also in opposition to "creeping privatisation" in the department, which is currently trialling the use of private firms to handle tax credit inquiries.
The strike will be followed by other forms of action such as a ban on overtime.
The union said around 75% of its 55,000 HMRC members were on strike, with support in some offices as high as 97% of members, including those working in Aberdeen (83%), Dundee (91%), Inverness (95%) and East Kilbride (97%); North Wales (95%), Cardiff (93%), Carmarthen (85%), and Swansea (80%); Bootle (96%), Chesterfield (90%), Dover (76%), Portsmouth (75%) and Stoke (90%).
PCS general secretary Mark Serwotka said: "It is hugely significant that this strike is taking place on the day the Prime Minister is threatening more welfare cuts, because it shows his Government prefers to target people on benefits instead of going after the tax dodgers.
"The case for investment instead of more cuts, as an alternative to austerity, could not be clearer than in HMRC where the money we collect funds all the other public services that we all rely on, and provides support to people when they need it."