Conservatives will legislate to make it more difficult for unions to call strikes in key public services if they win this year's election.
Industrial action in the health, education, transport and fire services would require the support of at least 40% of all those entitled to take part in strike ballots - as well as a majority of those who actually turn out to vote.
A Conservative source said this would prevent strikes going ahead on the basis of majorities in ballots in which only a small proportion of the unionised workforce has participated.
The move was denounced as a "democratic outrage" by the TUC, who said it would effectively end the right to strike in the public sector at a time when Conservatives are planning pay restraint and large-scale job cuts.
GMB general secretary Paul Kenny pointed out that Conservatives failed to secure 40% support of those who voted in the 2010 general election, while only 15 Tory MPs had the backing of 40% of those entitled to vote in their constituencies.
Under proposals to be included in the Conservative manifesto for the May 6 election, Tories also pledged to end the ban on the use of agency staff to cover for striking workers, and promised a review on the possible introduction of minimum service levels to ensure that core services remain available during strikes.
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