Commuters prepare to travel on the District Line of the London Underground which is running a limited service due to industrial action on April 30, 2014 in London, England Getty London (AFP) - Britain's Conservative government announced on Wednesday it would seek to pass a law restricting the conditions under which workers can stage strikes in a move that outraged the country's unions.

Ministers said the "Trade Union Bill", which follows a high-profile union shutdown of London's underground rail network last week, balanced the rights of businesses and workers but left-leaning daily The Guardian called it the "biggest crackdown on trade union rights for 30 years".

The proposed law would require a turnout of at least 50 percent of members in strike ballots in order to authorise action.

Proposed strikes in key public services such as health, education, fire, transport, border security and energy would also have to be backed by 40 percent of all members eligible to vote.

Currently, a simple majority of those balloted are required to vote in favour of a strike in order to trigger one, which critics say allows a small minority of workers to call strikes.

The law would make unions ask all of their members whether they wish to pay into the union's political fund -- usually raised automatically from trade union membership fees and a major source of funding for the opposition Labour party.

Business Secretary Sajid Javid said that the new rules would balance the rights of trade unions "with those of working people and business".

"These changes are being introduced so that strikes only happen when a clear majority of those entitled to vote have done so and all other possibilities have been explored," Javid said.

He said the bill would deliver on promises made by Prime Minister David Cameron, who vowed to tighten restrictions on striking in an election campaign that saw his centre-right party win a surprise majority in May.

- 'Unnecessary attack' -

Under the law, votes in favour of striking would become invalid once four months had passed.

"Safeguards" would also be introduced to ensure that workers who chose not to strike could "go about their business without fear of intimidation", the business department said in a statement.

The government is also considering limiting pickets permitted outside workplaces and repealing a ban on companies hiring agency staff to fill in for striking workers.

Trade unions reacted furiously to the news.

"This bill is an unnecessary attack on workers' rights and civil liberties that will shift the balance of power in the workplace," said Frances O'Grady, secretary general of the Trades Union Congress, which represents 52 unions.

"Getting a pay rise or defending terms and conditions will become far harder for working people."

The public services union Unison decried the proposals as "spiteful", while Paul Kenny of the Labour-affiliated GMB union accused the Conservative party of trying to undermine funding for its rival.

"It is clear that the Tory Party High Command intend to make the Labour Party bankrupt by cutting off the main source of funding that they have relied on since the 1930s," Kenny said.

But business groups welcomed the proposals, with the Confederation of British Industry praising minimum vote turnout rules as "important, but fair".

Adam Marshall, Executive Director of Policy and External Affairs at the British Chambers of Commerce, said that the right to strike "must be exercised with the greatest restraint".

"Businesses will see this as a sensible piece of legislation that carefully balances the rights of those wishing to withhold their labour, against the rights of those who rely on access to essential services," Marshall said.