The Conservatives are pledging to legislate to make it more difficult for trade unions to strike if they are returned to power in next year's general election.

The party's election manifesto will include a commitment that at least half the eligible union members must actually vote in a strike ballot for it to be lawful.

The law would also be changed to end "rolling mandates" for industrial action - with a firm, three-month limit on the mandate for action after a ballot is held.

The Conservatives say that they will also tighten the rules on picketing, making illegal picketing a criminal offence.

Cabinet Office Minister Francis Maude said the plans represented a "sensible and proportionate" package of reforms.

However the TUC said the proposals would make legal strikes "close to impossible" and were deliberately designed to hold down pay levels among workers.

The Tories say that the introduction of the 50% turn-out threshold would have prevented around two-thirds of the strikes which took place over the last four years.

In other measures, the party said that it would require unions to set out on the ballot paper the exact form or action they were proposing with a vote on each aspect of the dispute.

Currently members are only required to vote 'yes' or 'no' in answer to a question as to whether they are prepared to take part in strike action and/or action short of a strike.

Mr Maude said: "I've always said that unions can play a constructive role in the modern workplace.

"But I've also warned that the more that union leaders pushed for disruptive strike action without even persuading a majority of their members to vote, the stronger the case would become for changing the law."

Business Minister Matthew Hancock added: "For too long, businesses, commuters and families have put up with disruptive and unnecessary strikes.

"That's why the next Conservative government will change the law so that union bosses cannot call strikes without genuine support from their members."

However TUC general secretary Frances O'Grady said Britain already had some of the toughest strike laws of any democracy and Conservative plans would mark a fundamental shift in the balance of power in the workplace.

"These proposals are designed to make legal strikes close to impossible. Not a single MP would have been elected under these rules," she said.

"You cannot have proper negotiations between employers and unions without some power for the union side.

"Making strikes near impossible will fundamentally shift the balance of power in British workplaces in favour of the employer - and as union negotiations often set the pace for pay rounds, this will hit non-union workers as much as those in unions.

"The purpose of this is clear. It is to ensure that the fruits of recovery are reserved for the few and kept from the many."