The Government has been accused of "going for the jugular" over its plans for strike ballot thresholds and other "anti-union" measures.
The president of Unison said the UK already has some of the toughest anti-trade union legislation in the world.
Lucia McKeever told the union's national conference in Glasgow: "Now the Government seems to want to go for the jugular. The Tories have a package of specific attacks for trade unionists."
Activists' time off is being threatened, while arrangements for collecting union subscriptions from salaries are set to be attacked, she said.
She told the 1,000 delegates that the general election had been a "disaster" for the labour movement, warning that austerity measures would intensify.
Mrs McKeever, a nursing assistant from Armagh in Northern Ireland, said: "Working people and public sector workers will continue to pay an ever higher cost - a million public sector jobs set to be lost by 2020, a further crackdown on tax credits and other benefits for those of working age, and we are primed to expect yet another public sector pay squeeze.
"Public spending is set to be slashed down to its lowest level for decades - a systematic dismantling of our welfare state and blatant attempt to change the shape of the public sector forever."
The union leader also attacked the " idiocy" of tying future governments to generate surpluses, adding: "It is a shameless attempt to drag us back to Victorian times, with a complete disregard for the impact in terms of lack of growth, and the need to invest in infrastructure."
The Government is planning a Trade Union Bill so that 50% of members would have to vote in an industrial action ballot, and 40% of those entitled to take part would have to back action for any strike to be lawful.
A Business Department spokesperson said: "People have the right to know that the services on which they and their families rely will not be disrupted at short notice by strikes supported by a small proportion of union members.
"The right to strike is important but it is only fair that there should be a balance between the interests of union members and the needs of people who depend on their services."