Prime Minister Theresa May's Conservative Party is on course to increase its majority in parliament in Britain's election tomorrow (Thursday), two last-minute opinion polls showed.

May, who called the surprise election six weeks ago, saying she wanted to go into the Brexit negotiations from a position of strength, has seen her once-commanding lead over the Labour Party narrow sharply.

But polling firm ICM said its final pre-election poll showed May's lead had increased by one percentage point to 12 points. Rival pollster ComRes said the lead had narrowed by two points but still stood at a comfortable-looking 10 points.

ICM said its 46-34 lead for the Conservatives over Labour would give May a majority of 96 seats in parliament, up sharply from the working majority of 17 she has had until now.

The Independent newspaper, which commissioned the ComRes poll, said the 44-34 lead it gave May would leave her her with a majority of 74, bigger than any Conservative majority since the days when Margaret Thatcher was prime minister.

Both polls were conducted after a deadly attack by Islamist militants in London on June 3.

ICM and ComRes have tended to give the Conservatives bigger leads than other polling firms.

Four other polls were expected to be published later on Wednesday, including one from Survation which has put the Conservatives' lead as narrow as just one percentage point.

But investors took the latest findings as a sign May was likely to emerge victorious on Thursday, bolstering her before Brexit negotiations starting later this month.

GAMBLE

Sterling rose by nearly half a cent against the U.S. dollar after the ICM poll was published, easing jitters in financial markets about the possibility that Labour under its hard-left leader Jeremy Corbyn could pull off one of the biggest election shocks in modern British history.

ComRes Chairman Andrew Hawkins said May was in the clear after securing the support of around 2 million voters - worth about six percentage points in the polls - who had backed the anti-EU UK Independence Party in Britain's last election in 2015 and have warmed to her tough stance on negotiating Brexit.

"Despite Mrs May’s ratings taking a hit during the campaign, older voters in particular have stuck with her party and it appears that the electoral gamble is about to pay off," he said.

The four polls expected later on Wednesday will be published by companies which have given the Conservatives smaller leads than ICM and ComRes in recent weeks.

There also have been big differences in projections of how many seats the Conservatives are likely to win.

Polling firm YouGov estimated earlier on Wednesday that the party would fall more than 20 seats short of a majority.

The contrasting poll findings have added to scepticism among critics of polling who point to how the industry largely failed to accurately predict the outcome of the 2015 election and last year's referendum vote to exit the European Union.

However, surveys mostly showed a steady narrowing of the lead of May's Conservatives for much of the past three weeks following the publication of the parties' election policy pledges.