LONDON (Reuters) - British Prime Minister Boris Johnson looks on course to win Thursday’s election though the race has tightened markedly and he can no longer be sure of a majority, according to opinion polls published on the eve of the vote.

The Dec. 12 election has been described by all parties as Britain’s most important in memory, with Johnson calling for a big majority so he can swiftly pull Britain out of the European Union next month.

The main opposition Labour Party promises a new referendum on Brexit as well as a 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 with an elaborate survey that estimates the outcome in individual constituencies, cut its forecast for Johnson’s likely parliamentary majority by more than half to 28 seats. Two weeks ago it had forecast a majority of 68.

Possible outcomes range from a landslide victory for Johnson to a hung parliament with no party in control, YouGov said, given the possibility of “tactical voting” in dozens of closely fought constituencies to deny a Conservative victory.

“It could not be tighter,” Johnson, the face of the Leave campaign in the 2016 referendum, said on Wednesday when asked about the opinion polls showing a tighter race. 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.

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 Johnson’s Conservatives were on course to win 339 seats in the 650-seat House of Commons with 43% of the vote, versus Labour’s 231 seats with 34% of the vote.

Slideshow ( 5 images )

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 $1.3107, a drop of more than half a cent in Asian trading. In London it was last at $1.3117, down 0.3% on the day. Half a dozen opinion polls were due on Wednesday.

An Opinium poll showed Johnson’s lead over Labour had narrowed to 12 percentage points from 15.

BREXIT PARALYSIS

Johnson called the snap election to break what he said was the paralysis of Britain’s political system after more than three years of crisis over how, when or even if to quit the EU.

He has promised to “get Brexit done” by Jan. 31 if he wins a majority. Corbyn has said he will negotiate a new Brexit deal maintaining closer trade and financial ties with the EU, and then grant a new EU referendum.

Polls open at 0700 GMT on Thursday and close at 2200 when an exit poll will give the first indication of who has won.

Slideshow ( 5 images )

LANDSLIDE OR LOST?

While there have been countless polls in the run-up to the election, YouGov’s “Multilevel Regression and Post-Stratification Model” is seen as the most meaningful, offering a prediction of seats, rather than just vote share.

It crunched data from more than 100,000 interviews over seven days, using demographics, the specific circumstances in individual constituencies, and national statistics to come up with a projection.

“We absolutely cannot rule out the 2019 election producing a hung parliament - nor can we rule out a larger Conservative majority,” said Anthony Wells, YouGov’s director of political research.

The polling showed that Johnson’s main strategy to secure victory is to break down the so-called “Red Wall” of long-held, Labour seats across the Brexit-backing Midlands and northern England.

“Most of the Conservative gains are expected to be in the north, in urban West Midlands and former mining areas in the East Midlands and County Durham - concentrated in areas that voted to leave the EU,” Wells said.

Labour are forecast to lose 31 seats, taking them to the worst performance since 1983 when Labour leader Michael Foot lost to Thatcher.

The Scottish National Party are forecast to make gains to 41 seats. The anti-Brexit Liberal Democrats are forecast to win 15 seats and the Brexit Party none, YouGov said.

In late May 2017, just over a week before the June 8 election, YouGov’s model correctly projected Johnson’s predecessor Theresa May would lose the Conservative majority, even though she had been ahead in most polls.

(GRAPHIC: Understanding Brexit - here)