All 650 seats in the House of Commons are up for grabs Thursday when voters will pass judgment on a divisive election that will determine Britain's future with the European Union.

Although opinion polls have placed Conservative Prime Minister Boris Johnson in the lead, analysts suggest the gap is not wide enough to guarantee a majority in parliament.

Johnson urged supporters at a London rally on Sunday to learn from the 2017 election, which saw former Conservative prime minister Theresa May fritter away a decisive lead in the polls and end up with a divided parliament.

"We're now in the final furlong of this race, and that is when of course the horses can still change places," Johnson said before urging the group to "drive for the line".

"We're going to get there, but only if we work really hard," he said.

Johnson and members of his cabinet also hit the phones in a bid to gain support, making calls from the Conservative Party headquarters in central London. In one comical moment, Johnson offered his name to one potential voter, but felt the need to add: "I'm the prime minister."

Johnson's cautious message comes in part because Britain's traditional political alliances don't necessarily apply in this election. British voters' 2016 decision to leave the EU has sparked a major overhaul in British politics, and many voters will make their choice based on a candidate's position on Brexit alone, rather than on past political ties.

The Conservatives are also worried that people who voted to leave the EU won't turn out to vote because they are fed up with politicians' failure to carry out the 2016 referendum decision, said Matthew Goodwin, a visiting senior fellow at the Chatham House think tank.

The main opposition Labour Party has also been moving to shore up votes that it could have counted on in years past. Party leader Jeremy Corbyn campaigned on Sunday in Wales, a part of the UK that has been a Labour stronghold for decades. He also urged supporters to take nothing for granted in this vote.

"When you are working, knocking on doors in the rain ... over the next few days, remember what you're doing it for," he said as people cheered. "You're doing it for the next generation. You're doing it to change the course of political history in Britain. Go flat out!"

Johnson pushed to hold the December vote, which is taking place more than two years early, in hopes of winning a majority and breaking Britain's political impasse over Brexit. He has pledged if the Conservatives win a majority, he will get parliament to ratify his Brexit divorce deal and take the UK out of the EU by the current January 31 Brexit deadline.

Labour says it will negotiate a new Brexit deal, then give voters a choice between leaving on those terms and remaining in the bloc. Labour's eye-catching domestic agenda has included promises to nationalise key industries and utilities and give free internet access to all.

But the Labour campaign has been beset by allegations that Corbyn - a long-time champion of the Palestinians - has failed to stamp out anti-Jewish prejudice in the party.

Labour is also being challenged on how it obtained government documents on exploratory talks on a Brexit trade deal with the US. The documents had appeared on the social media platform Reddit for weeks before Labour brought the documents into the election debate just days ago.

But Johnson has troubles of his own on the question of whether Russia is meddling in the vote. Critics have been challenging the British government's failure to release a parliament intelligence committee report on previous Russian interference in the country's politics. They say it should have been made public before Thursday's vote.

Australian Associated Press