British Prime Minister Boris Johnson looks on course to win a general election though the race has tightened markedly, according to opinion polls published on the eve of the vote.
The 12 December election has been described by all parties as Britain's most important in memory, with Mr Johnson calling for a big majority so he can swiftly pull Britain out of the EU next month.
The main opposition Labour Party promises a new referendum on Brexit as well as renationalisation of utilities and railroads in its most left-wing platform for decades.
YouGov, which accurately predicted the outcome of the last election two years ago, has cut its forecast for Mr Johnson's likely parliamentary majority by more than half to 28 seats.
Two weeks ago it had forecast a majority of 68.
Possible outcomes from Thursday's poll range from a landslide victory for Mr Johnson to a hung parliament with no party in control, YouGov said.
Mr Johnson, the face of the Leave campaign in the 2016 referendum, said of the contest "it could not be tighter" when asked about the opinion polls on Wednesday. He delivered milk to voters before sunrise.
In a last-minute pitch for votes, both Johnson and Labour leader Jeremy Corbyn went on whistlestop tours of the country.
Mr Corbyn called on voters to reject the politics of despair and division he said the Conservatives had sown and urged Britons to vote for justice and equality.
The YouGov model indicated Mr Johnson's Conservatives were on course to win 339 seats in the 650-seat House of Commons with 43 per cent of the vote.
While that would be the Conservatives' best performance in seat terms since Margaret Thatcher's 1987 victory, YouGov said the Conservatives' tally could fall anywhere from 311 to 367.
Sterling fell sharply immediately after the poll, hitting as low as $US1.31 ($A1.91), a drop of more than half a cent in Asian trading.
An Opinium poll showed Mr Johnson's lead over Labour had narrowed to 12 percentage points from 15.
