British Prime Minister Theresa May is on course to increase her majority in parliament in Britain's election, with final opinion polls suggesting her gamble to call a vote to bolster her position in Brexit negotiations will pay off.

On the eve of the election, Ms May returned to her core message that only she can deliver a good Brexit deal, moving on from a heated national debate over security after two deadly Islamist attacks, while veteran hard-left Labour leader Jeremy Corbyn hit back at Conservative fiscal austerity.

Ms May has seen her once-commanding lead over the Labour Party and Mr Corbyn narrow sharply since she surprised almost everyone by calling a snap election in April.

But a final round of opinion polls suggested she would increase the small majority she inherited from David Cameron last year, shortly after the surprise referendum decision to take Britain out of the European Union.

Of five polls published on Wednesday, two showed the Conservatives widening their lead over Labour, two showed a narrowing and one was unchanged — but they suggested the Conservatives would increase their majority in parliament.

The polls were conducted after a deadly attack by Islamist militants in London on Saturday.

Talk of a landslide majority fades

The campaign has seen unexpected twists — the shrinking of Ms May's poll lead over the opposition Labour Party and attacks in Manchester and London that killed 30 people.

Pollsters still expect Ms May's Conservatives to win, although talk of a landslide majority of more than 100 seats has faded.

The polls started narrowing after Ms May launched a new policy on care for the elderly that proved unpopular.

She backtracked days later, prompting opposition parties to pour scorn on her claim to offer "strong and stable leadership".

Then came a suicide bombing at a pop concert in Manchester on May 22 that killed 22 children and adults, and a van and knife attack on London Bridge and in nearby Borough Market that killed eight people on June 3, five days before the election.

The attacks threw the campaign spotlight onto security and prompted questions from Ms May's opponents and media about her record overseeing cuts in police numbers during her years as interior minister from 2010 to 2016.

But the security issue was not seen as helpful to Mr Corbyn, who has voted against counter-terrorism legislation and expressed reservations in the past about police shoot-to-kill tactics.

May and Corbyn return to core messages

On Wednesday, the eve of the vote, Ms May tried to bring the campaign back to Brexit.

"When it comes to the election tomorrow, I think the choices and the questions that people need to ask are exactly the same today as they were right at the beginning of the campaign," she told a campaign rally in Norwich, eastern England.

"And the first is a question of who do you trust to actually have the strong and stable leadership that is going to deliver the best deal for Britain in Europe?"

Ms May has sought to portray Mr Corbyn as the weak leader of a spendthrift party which would crash Britain's $2.5 trillion economy and lead the country to ruin in chaotic Brexit negotiations.

Mr Corbyn, a veteran left-winger who unexpectedly won the Labour leadership in 2015 after three decades on the party fringe, has hit back that Conservative fiscal austerity has hurt the poor and increased social inequalities.

A crowd of Jeremy Corbyn supporters at a campaign event. ( Reuters: Peter Nicholls )

Labour propose to build a fairer society through policies such as increasing tax for the richest 5 per cent, boosting workers' rights, scrapping university tuition fees and investing 250 billion pounds in infrastructure.

"The choice is quite simple. Five more years of a Tory government, five more years of austerity, five more years of cuts. Or something different," Mr Corbyn told supporters in Colwyn Bay, north Wales, to cheers and applause.

For Ms May, the challenge is not only to win but to surpass handsomely the 12-seat majority her Conservative predecessor David Cameron won in 2015.

A narrow victory would undermine her authority both inside her party and at talks with the 27 other EU leaders.

Violence dominates final weeks of campaign

London police were still dealing with the fallout from Saturday night's attack, when three jihadis drove a van into pedestrians on London Bridge before running into narrow streets bustling with nightlife and began stabbing people.

Security has been increased nationwide and the violence has dominated the final weeks of campaigning.

Ms May responded to the attack with a pledge to crack down on Islamist extremism and strengthen police powers.

"If human rights laws get in the way of doing these things, we will change those laws to make sure we can do them," she said in an interview with the Sun.

The head of campaign group Amnesty International said the pledge was outrageous and a gift to autocratic leaders around the world.

Two of the three London Bridge attackers, who were all shot dead at the scene by police, had been known to authorities beforehand.

Italy said it had flagged Youssef Zaghba as a potential risk after he moved to England last year, while Khuram Butt was known to British security services.

Reuters