Would-be voters have been leaving polling stations before casting their vote due to long queues.
Waits of more than half an hour have been seen across England on Thursday morning.
Queuing appeared particularly widespread in London, with long lines in a number of constituencies.
Chris Schofield, in Bermondsey and Old Southwark, said more than 70 voters were waiting, 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.
“Atmosphere is very London: orderly queueing and no-one is talking to each other.”
Several voters said they had never seen queues like it.
Schofield added: “I think it’s the election of a lifetime for many of us.”
Alixe Bovey had been queueing for 35 minutes in the Streatham constituency.
“In 20 years of voting in Streatham Hill, always at about this time of day, I have never encountered a queue of more than six or seven people,” she tweeted.
Waits were also reported in English cities such as Cambridge, where John Walsh tweeted to say it was the “first time ever” he had to queue.
In Bermondsey, south-east London, a burst water main caused deep flooding in the area, leading some voters to decide to leave and return later.
Graham Kings said: “I could have gone home and put wellington boots on and waded across the flooded road to try to get in, but I had to go to work so will vote this evening.”
Some voters said they were encouraged by the queues, suggesting it could mean a greater turnout than the last general election.
In the 2017 poll, there was a turnout of 66.4%.
Voters unable to vote for whatever reason can return to their polling stations at any time before 10pm on Thursday evening.