Unison Cymru called on Welsh ministers and council leaders today to take a firm stand on tax dodgers by banning them from getting their grubby hands on public contracts.
The union's demand follows yet more reports of corporate giants such as Google, Amazon and Starbucks coughing up tiny amounts of tax.
It contrasts that with the deep cuts expected to be contained in the Welsh government's next budget.
Unison's head of local government in Wales Dominic MacAskill said: "It is entirely wrong that companies seeking to avoid paying their fair share of tax should be awarded public contracts.
"Public bodies in Wales spend hundreds of millions of pounds annually through procurement.
"Government and council procurement policies offer ways to use that spending to deliver local social, economic and environmental benefits.
"Public bodies in Wales should follow the example of European cities such as Helsinki and Paris, which are already acting against companies using tax havens."
He added that a similar approach should be taken to ensure better rights and working conditions, extend the living wage to people working for private contractors and exclude firms that blacklist trade unionists.