Britain’s coalition government was on Thursday set for a collision between its two partners — the Tories and the Liberal Democrats — as people voted in a referendum to decide whether to retain the first-past-the-post voting system or switch to Alternative Vote (AV) in which voters rank candidates in order of preference.

Opinion polls gave the "no" camp, backed by the Tories, an almost unassailable lead over "yes" campaigners led by the Lib Dems.

Elections were also held for the Scottish Parliament, devolved assemblies in Northern Ireland and Wales, and hundreds of local bodies across England. Both Tories and the Lib Dems were expected to be routed in these elections.

But all eyes were on the referendum whose outcome, whichever way it goes, will have a bearing on the unity of the coalition already under pressure after a campaign marred by personal insults between senior figures in the two parties.

A defeat for the "yes" campaign will be a huge personal setback for the Lib Dem leader and Deputy Prime Minister Nick Clegg already deeply unpopular in his party for propping up a Tory Government. If, on the other hand, the Lib Dems win the vote there will be pressure on Tory leader and Prime Minister David Cameron from party hardliners who never thought the referendum was a good idea.

The Opposition Labour party has a foot in both camps with its leader Ed Miliband backing a change while nearly half the party, including some senior figures, in the opposite bloc.

It is Britain’s first nationwide referendum in more than 30 years. The last was held in 1975 over Britain’s continued membership of the European Common Market and it was won by the "yes" camp. Since then, referendums have been held in Scotland, Northern Ireland, Wales and London but not across the United Kingdom.

Results are expected late on Friday.

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