Counting is under way after polls closed in the most significant electoral test for the Government since the formation of the coalition a year ago.
If recent surveys are borne out, Deputy Prime Minister Nick Clegg's Liberal Democrats look set to be the big losers of the night, while Labour leader Ed Miliband will be hoping to win hundreds of new councillors and David Cameron's Conservatives are expected to suffer some setbacks.
The Lib Dems are braced for heavy losses across a swathe of elections for English local authorities and the devolved legislatures for Scotland, Wales and Northern Ireland.
And final opinion polls suggest the referendum on adopting the alternative vote (AV) system for Westminster elections - which the Lib Dems strongly support - is heading for an overwhelming defeat for the Yes campaign when the result is announced late on Friday.
A close finish is expected in Scotland between Labour and the Scottish National Party, while in Wales voters are waiting to see whether Labour can regain its overall majority after four years of coalition with Plaid Cymru. Results in the Northern Ireland Assembly elections are not due until the weekend.
Voting in his Sheffield constituency, Mr Clegg appeared to acknowledge that his party would pay a price for supporting unpopular coalition cuts, saying: "Lots of people have got, obviously, questions and some people have got objections to what the Government is having to do.
"But I think most people - the vast majority of people - accept that we're having to do a difficult job in difficult circumstances and that we're trying do it as fairly and compassionately and responsibly as possible."
With the Labour Party divided over electoral reform, there are already signs that Mr Clegg is being lined up as the scapegoat for the failure of the Yes to AV campaign. In the Commons, Labour MPs mocked the Lib Dem leader, suggesting the Yes campaign had suffered from a "dead Clegg bounce" as a result of his unpopularity.
Conservative ministers are preparing to rebuild bridges with Lib Dem colleagues after a bruising referendum campaign which has threatened to strain tensions within the coalition to breaking point.
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Leader of the Commons Sir George Young acknowledged that there are "tensions" between the coalition partners, but insisted they are nothing like as bad as those which racked Labour during the Blair-Brown years.
