The head of Britain’s biggest public sector union has threatened to break the law by holding illegal strikes to fight against job losses and pay restrictions, if the Government tightens the rules to make it harder to hold industrial action.

Dave Prentis, leader of the 1.3-million-strong Unison, warned in an interview that “nothing is ruled out” in the long conflict against a “vindictive” and “hostile” government, which could produce the kind of confrontation between government and unions not seen since the 1980s.

In a comment likely to dampen the spirits of Labour activists, he told The Independent that the union is gearing up for a ten-year battle against a “hostile” government – indicating he doubts the chances of Labour recovering enough to win the next general election in five years.

Union leaders are furious about planned legislation to withdraw legal protection from any union that calls a strike supported by fewer than 40 per cent of all those eligible to vote.

Such a move threatens to cripple Unison’s ability to fight an austerity programme, which they fear could see 800,000 public sector jobs disappear by 2019, in addition to the 400,000 already axed since 2010.

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Unison, the main union for local government and NHS staff, called 700,000 council employees and 400,000 health workers on strike last year – often on the back of ballots in which turnout was below 25 per cent.

“This isn’t an issue of trade union rights. This is an issue of civil liberty,” Mr Prentis said ahead of Unison’s national conference in Glasgow, which opens tomorrow.

“It is totally undemocratic, and we will resist it. We will fight it in the Commons with all our means. We will fight it in the House of Lords as well. If need be, we will seek judicial reviews, and we will take claims to Europe.

“Industrial action for us is the last resort, but we are not going to have employers being able to act in brutal ways and our people not to be able to stand up for themselves. If it means taking unlawful action, that is something we will have to look at – because it’s the law that has moved against us.

It’s not us seeking to break the law. We have been put in this position. We’re not ruling anything out. Why should we?”

Prentis suspects that the Conservatives never seriously meant to bring in their planned new union law, but put it in their election manifesto believing that they would be back in coalition again with the Liberal Democrats, who would block it.

Speaking ahead of his speech to a union conference on Tuesday, he added: “It is vindictive. What they are saying is that anybody who abstains is voting ‘no’. Apply that to police commissioners, and we wouldn’t have any. Apply that to many MPs, and they wouldn’t be elected. It’s a 1970s-type attack on trade unions.”

Part of the union’s grievance is that they are forced by law to conduct ballots by post, in which members have to reply by post. They want to be allowed to introduce telephone and online voting, or the use of secure terminals placed in offices at the union’s expense, to increase turn out.

Next month Unison will hold a special conference of union members who are also members of the Labour Party to decide whether to back any of the current candidates for the Labour leadership. They may decide to endorse none and let their members make up their own minds. Mr Prentis said the issue could depend on whether there is a candidate offering a “real alternative – or are they fifty shades of grey?”

One issue before the Unison’s ruling executive is whether Mr Prentis, one of the TUC’s longest serving union bosses, will be there to lead the fight. His Wikipedia has been edited to make him appear younger than he actually is – 67 – and his third five-year term as general secretary ends in December.