THE Conservatives yesterday unveiled plans to make it far harder for public-sector workers to go on strike, provoking a wave of criticism from trade unions and other political parties.

The Tory manifesto for May's General Election is to include a pledge on a 40% support threshold for strikes in "core services" affecting health, education, transport and fire services.

At present, strikes are valid if backed by a simple majority in a ballot, regardless of turnout.

Under the Tory plan, 40% of all those eligible to vote, as well as a majority of voters, would have to support a strike for it to go ahead.

A 40% threshold would have made almost three-quarters of significant strike ballots in the last four years invalid.

Union leaders condemned the plan as a "democratic outrage". They pointed out the Tories failed to secure 40% of the vote in the 2010 election, and only one in 20 Tory MPs entered Parliament with the support of 40% of their local electorate.

Employer groups the CBI and the British Chambers of Commerce backed the threshold.

The Tories also said if elected with a majority on May 6 they would end the ban on the use of agency staff to replace striking workers, which would slash union bargaining power.

There would also be a study of introducing minimum service levels to ensure core services did not shut completely during a strike.

The proposed changes come on top of a recent requirement for a 50% turnout in strike ballots.

Transport Secretary Patrick McLoughlin said the legislation would be passed in the first year of a Tory government and would stop "politicised union leaders" holding the country to ransom.

"It is only fair that the rights of unions are balanced with the rights of hard-working taxpayers who rely on key public services," he said.

"As part of our long-term economic plan for our country, we want commuters, parents and families to be able to get on with their day with as minimal disruption as possible."

But TUC general secretary Frances O'Grady said a 40% threshold would "effectively end the right to strike in the public sector".

She said: "No democracy ­elsewhere in the world has this kind of ­restriction on industrial action.

"We know they plan to get rid of a million public sector jobs and cut the value of public-sector pay every year in the next Parliament if they win the election. Now they are also going to make it impossible for public-sector workers to resist."

Unison general secretary Dave Prentis said: "These measures would make it virtually impossible for anyone in the public sector to go on strike - shifting the balance completely in favour of the ­Government and employers."

Unite general secretary Len McCluskey said the planned changes would have a "chilling" effect.

"This speaks volumes about the sort of government we could expect from the Tories."

The 40% threshold mirrors the notorious 40% rule introduced by Labour MP George Cunningham in the 1979 referendum on a Scottish Assembly.

Most Scots who voted in the ­referendum backed an Assembly, but because less than 40% of the entire electorate had supported it, the Yes side lost and devolution was shelved for 18 years.

Roseanna Cunningham, the SNP Cabinet Secretary for Fair Work, said: "This is a typical Tory attack on the rights of workers that will not benefit business or the public sector one little bit."