The first votes in the general election could be cast at 7am today, with the final votes accepted at 10pm.
In some parts of the country, members of the public have been reported leaving polling stations before casting their votes due to long queues outside.
Waits of more than half an hour were reported at various locations across England this morning.
Queuing appeared particularly widespread in London, with long lines reported in a number of constituencies.
Posting on Twitter, Summer Ellis said: “Big queue for Sheffield Central at 10.30.”
Danny Antrobus said there was a ‘steady stream of people coming and going’ at his polling station in the Sheffield Brightside and Hillsborough constituency.
Also posting on Twitter, Annabel G, who voted at the polling station at Wildcapes on Stafford Road, Norfolk Park, said: “Not much from the outside but the first time I've ever had to queue inside to vote in Sheffield. Age range too.
“Also I say it every year but if you're a woman and you don't vote today remember women literally died fighting for your right to do so. Don't accept being voiceless.”
Joe Buckley said there were ‘queues in various polling stations around Sheffield Hallam’ and Anne Hollows said there are queues to vote in Millhouses.
Elsewhere, Chris Schofield said more than 70 voters were waiting to vote in the Bermondsey and Old Southwark constituency - some of whom gave up and left during his 20-minute wait.
"It's about 20 times busier than it was in 2017, and for the locals and Euro elections," the 27-year-old consultant said.
"I think it's the election of a lifetime for many of us."
Alixe Bovey reported queueing for 35 minutes in the Streatham constituency.
Many members of the public said they were encouraged by the queues, suggesting it could mean a greater turnout than in the last general election.
In the 2017 poll, there was an electorate of 3,988,490 people and a turnout of 66.4 per centre.
The Electoral Commission said polling stations ‘can get very busy, particularly towards the end of the day,’ but stressed that voters in a queue before 10pm will be entitled to apply for a ballot paper.