The country's largest trade union has called on the Government to drop the 100 euro household charge until a fully fledged and fair property tax is imposed.

Siptu general secretary Jack O'Connor stopped short of urging homeowners not to pay the levy and instead urged a new system to exempt the lowest paid.

"It's not our intention to call on people not to pay it. We are very reluctant in the first instance to disobey the law unless in the most extreme circumstances," he said.

He said he would not encourage civil disobedience without being able to guarantee that those who refuse to pay will be protected by the union.

Left wing and Independent TDs have been campaigning for 1.6 million homeowners eligible to pay the charge not to register.

Richard Boyd Barrett, People Before Profit TD and opponent of the charge, said there is mass opposition to it: "This is a flat tax being imposed on people already struggling to pay rent, pay mortgages and scrape money together for basics like utilities and, in many cases, food."

Siptu's national executive council called for a fair, progressive and proportionate property tax which asks wealthier households to pay more.

It said: "The Household Charge as currently proposed by the Government is a flat tax which is unfair and regressive in that it subsidises wealthy people at the expense of middle and low-income families."

It added that those on lower incomes should be exempt from a property tax: "The way the tax is currently being implemented is playing into the hands of those wealthy and vested interests who oppose the very principle of a fair and progressive property-tax system."

Mr O'Connor said the flat-rate household charge is playing into the hands of powerful lobbyists.