By Peter Griffiths
LONDON (Reuters) - Prime Minister David Cameron ordered rebellious lawmakers on Thursday to back his plan for a law guaranteeing a vote on Britain's European Union membership as he sought shore up his party, his leadership and the coalition.
Conservative rebels have been pushing him to take a hard line on Europe, resurrecting party splits that contributed to the downfall of former prime ministers Margaret Thatcher and John Major in the 1990s. An election is due in 2015.
Cameron's promise in January to claw back powers from the EU and then put Britain's membership of the bloc to a vote by 2017 failed to silence eurosceptics and halt the rise of the anti-EU UK Independence Party.
He bowed to pressure this week when he agreed to guarantee his pledge in law, despite opposition from pro-EU Liberal Democrats coalition partners whose leader warned it would be a "calamitous mistake" to leave Britain's biggest trading partner.
Liberal Democrat and Labour opposition and a lack of parliamentary debating time mean it may never become law, and any new law could be repealed by the next government.
Conservatives remain keen to debate the bill in parliament to show voters that they are serious about holding a referendum.
That has strained relations within the coalition and within the party. More than 100 Conservatives, a third of the total, criticized him in parliament on Wednesday over his stance.
Seeking to draw a line, Cameron said all Conservatives must support the referendum law in parliament. Party enforcers will impose a "three-line whip", their strictest order for lawmakers to back a vote or face disciplinary action.
The opposition Labour Party, which has a 10-point poll lead, said the Conservative party was in chaos, led by eurosceptic members who see the EU as a wasteful "superstate" that threatens Britain's sovereignty.
"This is a prime minister who has lost control of the agenda and lost control of his party," said Labour foreign affairs spokesman Douglas Alexander.
The referendum bill is not a government proposal because it is opposed by the Liberal Democrats. Instead, Conservative James Wharton will propose the bill in a personal capacity.
Deputy Prime Minister and Liberal Democrat leader, Nick Clegg, said the government should focus on fixing the economy.
Tensions between EU nations in and outside the euro zone were highlighted on Thursday when French President Francois Hollande called for a euro zone economic government with its own budget, a harmonized tax system and a full-time president.
(Additional reporting by William James; Editing by Louise Ireland)