SIPTU has called on the Government to suspend the Household Charge, calling it unfair and regressive.
The decision was taken by the union's National Executive Council.
The Government expects to raise €160m from the charge this year.
SIPTU unanimously endorsed a motion stating that the charge, in its current form, subsidised wealthy people at the expense of middle and low income families.
In a statement, it said it "supports the principle of a fair and progressive property tax which is proportionate and which recognises that wealthy households can afford to pay more than those with modest earnings".
The union said those on modest incomes should be exempt.
It called for a suspension of the Household Charge until it was replaced by what it called a fairer and progressive tax.
SIPTU also called on the Government to suspend existing property tax reliefs and to introduce a solidarity levy on those earning over €100,000, which it said would make up for lost revenue from the Household Charge.
A Government spokesman said the charge was a "temporary" measure that will be replaced by a more progressive and fairer charge.
He said the Government was "required to address the gap in the nation's finances".