The boss of the largest public sector union will tell the government this week that its efforts to crush union power will fail, after a successful recruitment drive in newly privatised businesses.

Dave Prentis, the general secretary of Unison, said an exodus of members following steep job cuts in the public sector had been all but made up for by new members recruited from businesses taking on public sector contracts.

Speaking ahead of Unison’s national conference in Glasgow, which opens on Tuesday, he revealed that 170,000 new members joined Unison last year, including 4,400 in the week of the general election in May, while 180,000 had left the union after they were made redundant.

He also attacked proposed legislation making strikes unlawful unless 50% of those entitled to vote take part in a ballot, with a threshold of 40% support of everyone eligible to vote needed in public services.

Prentis said the move was vindictive and the union could mount a legal challenge, but he maintained the new law would make no difference to the union’s strategy on industrial action.

“If the government is going to take away people’s right to organise and protest it is a civil liberties issue,” he said.

Unison has supported a succession of strikes over the last five years in protest at cuts in jobs, pay and pensions under a strategy Prentis said would need to undergo a thorough review.

“We must review everything we do. It would be tragic to behave like first world war generals, leading people over the top only for them to be shot down. We must choose our battles carefully.”

He plans to slim down the number of campaigns the union supports from the current 55 and link up with campaign groups to show how cuts in public service provision damage communities and abandon the most vulnerable.

“Local community services have been hit particularly hard. We can’t have care of the elderly and vulnerable at the levels of funding being proposed by ministers. To say that you can cut public services again and again and nobody suffers is just wrong.

The union is expecting public sector employers in the health service and local government to experience acute skills shortages as the government cuts pay and conditions. He said the NHS was already suffering a shortage of paramedics among many other areas, forcing health trusts to rely on agencies and foreign recruitment.

“We are already stealing nurses from countries that can barely afford to train them, which is outrageous, and that is only going to get worse,” he said.

Unison is a Labour-affiliated union with around 430,000 members eligible to vote in the forthcoming leadership ballot, with a special conference being held next month before the union decides whether to recommend one of the four contenders.

Prentis, who has been scathing about Labour infighting and its failure to engage with ordinary workers, said it was possible the union would decide not to recommend anyone, a position that could also be adopted by other unions.

