Conservative plans to crackdown on strike action by setting tough new voting thresholds have been labelled an attack on ‘democratic and legitimate rights’ by unions.

At least half of balloted workers would have to vote for the industrial action under the Tory manifesto plans - while in transport, health, education and fire, strikes would require the support of at least 40% of those entitled to vote as well as a majority of those who cast a ballot.

‘This turnout threshold will be an important and fair step to rebalance the interests of employers, employees, the public and the rights of trade unions,’ the document states.

It adds: ‘We will also repeal nonsensical restrictions banning employers from hiring agency staff to provide essential cover during strikes; and ensure strikes cannot be called on the basis of ballots conducted years before.’

Len McCluskey, Unite general secretary, accused the Tories of trying to ‘turn the clock back to the era of Margaret Thatcher and her union-smashing agenda’.

‘When it comes to electoral credibility and thresholds, David Cameron must be a little red faced when so many of his own cabinet and ministers failed to win 40% of the vote in the 2010 election,' he said.

‘The fact is that trade unions in the UK already face some of the most restrictive laws in Europe. If David Cameron wants to increase participation in ballots then he should join us in demanding the right to secure online voting and the ability for workers to vote in their workplaces.’

There is currently no minimum voting threshold for strike action in the UK.