British Prime Minister Theresa May is on course to lose her overall majority in the British general election, according to an exit poll released as soon as voting finished Thursday night.

The poll, commissioned by Britain's main broadcasters, showed May's Conservatives would have 314 seats, losing 17 seats and the overall majority she had before the election. She needs to win at least 326 seats to claim an overall majority in the House of Commons, Xinhua reported.

May's main rivals, Labour, would have 266 seats, according to the exit poll, which gave the first clue about the likely outcome of the snap general election called just a few weeks ago by May.

The minority Liberal Democrats would gain six seats to reach 14, the Scottish Nationalists will lose a third of their seats, getting 34, and the Greens would hold on to their single seat in Westminster.

It will be early hours Friday before the result is confirmed, but similar exit polls carried out at the previous two general elections in Britain came very close to predicting the results correctly.

In the 2015 general election, which came after five years of a general election between the Conservatives and the Liberal Democrats, few polls predicted a majority win for then prime minister David Cameron.

The poll predicted not only would Cameron win, but also that his coalition partners since 2010, the Liberal Democrats would be massacred.

The 2015 exit poll predicted nine seats for the Lib Dems, and in the event they ended up with eight, losing around 50 MPs.

Counting of millions of votes started soon after ballot boxes were delivered to counting stations, with the first declarations likely before midnight.

The vast majority of results will be declared between 2 a.m. and 4 a.m. local time, with May and her main rival, Labour leader Jeremy Corbyn, eagerly waiting to see which party, if any, crosses the 326-seat threshold. That is the number of seats needed to guarantee a majority in the 650-seat House of Commons.

Labour's Emily Thornberry, the shadow foreign secretary, said late Thursday night May should consider her position if the exit poll proves accurate.

Speaking on Sky news, Thornberry said: "I think she should go. I think we're (Labour) on the verge of a great result." Labour's Shadow Chancellor John McDonnell also said he thought May's position would be untenable on the result of the exit poll.

May's expected losses come after the 100-plus wins predicted early on in the campaign.

In the run-up to the general election May had insisted that there would be no general election until the parliamentary term ended its five-year span in 2020.

She changed her mind, saying she wanted more support in her negotiations over the terms for Britain leaving the European Union.

Brexit, though, has not dominated the election campaign which was overshadowed by Britain's worst terrorist strike since 2005, with the detonating of a bomb by a suicide bomber at the end of a concert by singer Ariana Grande at the Manchester Arena.

Then came a second attack on London Bridge by three terrorists who were shot and killed within minutes by armed police.

May's manifesto also included controversial plans to cut heating allowances to pensioners and also reducing the guaranteed annual increase in their pensions. It also included a move that would see using the value of their homes to pay for care in their old age, dubbed by critics as the dementia tax.

The exit poll will also provide some comfort to Labour leader Jeremy Corbyn. At the start of the campaign a landslide of Labour losses had been predicted, weakening Corbyn's role as leader. But in the final weeks of the campaign the gap between the two leaders narrowed. For Corbyn the gap was a bridge too far to cross, but it is likely to weaken the hand of his critics, including many of her own MPs, who have wanted him to go.

Political commentators are already saying May's gamble in calling a snap election will turn out to be a disaster for her if the actual results reflect the exit poll.