STRIKES would be "close to impossible" under a Tory plan to make illegal any ballot that did not have at least half of eligible workers participating in it, trade union chiefs have warned.

In a pre-election move, Conservative HQ signalled the party also intended to change the law to end "rolling mandates" for industrial action with a three-month limit on any approval for action after a ballot was held and that illegal picketing would be made a criminal offence.

Francis Maude, the Cabinet Office Minister, said the proposals were a "sensible and proportionate" package of reforms.

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The Tories pointed out the introduction of the 50 per cent turnout threshold would have prevented around two-thirds of the strikes which had taken place during the last four years.

In other measures, the party said it would also require unions to set out on the ballot paper the precise form of action being proposed with a vote on each aspect of the dispute. At present, union 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.

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

"But I've also warned the more 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."

But Frances O'Grady, the TUC General Secretary, said: "The purpose of this is clear, it is to ensure the fruits of recovery are reserved for the few and kept from the many."

Michael Dugher, for Labour, branded the package as "desperate stuff".