The contest, the first to be held in December in nearly 100 years, follows those in 2015 and 2017.
Polling stations in 650 constituencies across England, Wales, Scotland and Northern Ireland opened at 07:00 GMT.
After the polls close at 22:00 GMT, counting will begin straight away. Most results are due to be announced in the early hours of Friday morning.
A total of 650 MPs will be chosen under the first-past-the-post system used for general elections, in which the candidate who secures the most votes in each individual constituency is elected.
In 2017, Newcastle Central was the first constituency to declare, announcing its result about an hour after polls closed.
Elections in the UK traditionally take place every four or five years. But, in October, MPs voted for the second snap poll in as many years. It is the first winter election since 1974 and the first to take place in December since 1923.
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Anyone aged 18 or over is eligible to vote, as long as they are a British citizen or qualifying citizen of the Commonwealth or Republic of Ireland and have registered to vote. Registration closed on 26 November.
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Details about where to vote can be found on the Electoral Commission website and are also listed on polling cards sent to households.
People do not need a polling card to be able to vote but will need to give their name and address at their local polling station. People can only vote for one candidate or their ballot paper will not be counted.
Prime Minister Boris Johnson has cast his vote - he visited a polling station in Westminster, taking his dog, Dilyn, along with him.
Labour leader Jeremy Corbyn, Liberal Democrat leader Jo Swinson, SNP leader Nicola Sturgeon, Green Party co-leader Jonathan Bartley and Plaid Cymru leader Adam Price are all expected to visit polling stations this morning. Brexit Party leader Nigel Farage has voted by postal vote.
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Ahead of the poll, the elections watchdog has reminded voters that taking selfies and other photos inside polling stations is not permitted and may be a breach of the law.
Many people have already put a cross next to the name of their favoured candidate by voting by post - more than seven million people used a postal vote two years ago.
Those who applied for a postal vote but have yet to return it to their Electoral Office must do so by 22:00. Alternatively, they can hand it into their local polling station by the close of polls.
According to the BBC's weather forecast, Thursday will be a wet day in many parts of the country, with highs of five degrees Celsius in Edinburgh, nine in Cardiff, seven in Belfast and eight in London.
Follow election night on the BBC
- Watch the election night special with Huw Edwards from 21:55 GMT on BBC One, the BBC News Channel, iPlayer
- As polls close at 22:00, the BBC will publish an exit poll across all its platforms, including @bbcbreaking and @bbcpolitics
- The BBC News website and app will bring you live coverage and the latest analysis throughout the night
- We will feature results for every constituency as they come in with a postcode search, map and scoreboards
- Follow @bbcelection for every constituency result
- From 21:45 GMT, Jim Naughtie and Emma Barnett will host live election night coverage on BBC Radio 4, with BBC Radio 5 live joining for a simulcast from midnight
https://www.bbc.com/news/election-2019-50731632
