Commuters at Earls Court underground station, awaiting the arrival of a train, attempt to complete their journey on Wednesday evening, in London, Britain, July 8, 2015. REUTERS/Peter Nicholls

By Kylie MacLellan

LONDON (Reuters) - The British government published plans on Wednesday for the biggest shake up of strike laws since the Thatcher era 30 years ago, setting itself up to battle trade unions over proposed limits to walkouts at key public services.

Prime Minister David Cameron, who last week described a 24-hour stoppage on the London Underground rail network as "unacceptable and unjustified", promised the curbs in his re-election campaign earlier this year.

Business lobby groups welcomed the plans while the Trades Union Congress said they would make legal strikes "close to impossible".

One union warned a rule requiring union members to make an active choice to pay into political funds would bankrupt the opposition Labour Party, as unions are its biggest financial backers.

"People have the right to expect that services on which they and their families rely are not going to be disrupted at short notice by strikes that have the support of only a small proportion of union members," said employment minister Nick Boles in defending the plans.

The new measures will require a turnout of at least 50 percent in ballots for industrial action. In key sectors such as health, transport and education, strikes will also need support from at least 40 percent of those eligible to vote.

Currently there is no minimum threshold for turnout and only a simple majority is needed to back action.

The last significant assault on trade union powers came under Margaret Thatcher, one of Cameron's Conservative predecessors, whose 1985 defeat of the miners after a bitter year-long strike was a defining moment of her premiership.

Len McCluskey, general secretary of Britain's biggest union Unite, said the government was trying to cast unions as "the enemy within".

"These measures aim to deny working people a voice and to tilt power still further towards the rich and big business, who funded the Tory (Conservative) re-election campaign," he said.

The RMT transport union, whose members joined the rail strike that caused travel chaos in London last week, said the government was trying to "shackle" the unions.

"The trade union movement will unite to fight this brutal assault on the most basic of human rights," said RMT General Secretary Mick Cash.

With his surprise majority in the May general election, Cameron is likely to gather enough parliamentary backing to turn the proposals into law.

Andy Burnham, the bookmakers' favourite to become the next Labour leader following Ed Miliband's post-election resignation, described the plans as politically motivated, while Labour's business spokesman Chuka Umunna said they were "divisive".

"The bill tries to drive a false wedge between government, industry, employees and the public by restricting rights, and at worst criminalising, ordinary working people," Umunna said.

(Editing by Tom Heneghan)