British Prime Minister David Cameron's Conservative Party says it will introduce tough strike laws for some public service workers if it wins the general election in May, prompting anger from unions that called the proposals an affront to democracy.

Under the Conservatives' plans announced Saturday, a strike involving health, transport, education or fire service workers would require the backing of at least 40 percent of eligible union members to be legal.

The move comes after a series of strikes last year by public sector employees, including teachers and firefighters, and stoppages by rail workers on London's underground train network that caused chaos for millions of commuters. Many of these strikes would have fallen foul of the new proposals. Cameron has previously argued industrial action without proper backing is unjustified.

With the opposition Labour Party running neck and neck with the Conservatives in opinion polls, the latest proposal is also designed to put pressure on Labour leader Ed Miliband, whom Cameron accuses of being in thrall to powerful unions.