Opinion polls put Starmer's party on course for a landslide victory but also suggest many voters simply want change after a period of infighting and turmoil under the Conservatives that led to five prime ministers in eight years.
This means Starmer, a 61-year-old former human rights lawyer, could take office with one of the biggest to-do lists in UK history but without a groundswell of support or the financial resources to tackle it.
"Today, Britain can begin a new chapter," Starmer told voters in a statement on Thursday.
"We cannot afford five more years under the Conservatives. But change will only happen if you vote Labour."
The country's 40,000 polling stations opened at 7am and closed at 10pm.
Sunak, 44, voted early with his wife Akshata Murty in the electoral district of Richmond in northern England that he represents in parliament.
Starmer cast his ballot later on Thursday morning with his wife in his north London constituency.
Prime Minister Rishi Sunak's Conservative Party has been in power in the UK for 14 years. (EPA PHOTO)
Hundreds of communities were locked in tight contests in which traditional party loyalties come second to more immediate concerns about the economy, crumbling infrastructure and the National Health Service.
Having called the election months earlier than expected, Sunak has in recent weeks abandoned his call for a fifth consecutive Conservative victory, switching instead to warning of the dangers of an unchallenged Labour Party in parliament.
Sunak issued a fresh rallying cry to voters for election day, saying a Labour government would hike taxes, hamper economic recovery and leave the UK more vulnerable at a time of geopolitical tension, charges Labour deny.
I've changed the Labour Party. — Keir Starmer (@Keir_Starmer) If you put your trust in me by voting Labour, I will change the country.July 4, 2024
"They will do lasting damage to our country and our economy - just like they did the last time they were in power," Sunak said on Thursday.
An exit poll after 10pm will give the first indication of the outcome.
Detailed official results are expected in the early hours of Friday.
If Labour are handed a blank cheque, they will use their supermajority to tax you more for the rest of your life.— Rishi Sunak (@RishiSunak) July 4, 2024
Labour has not set pulses racing with its pledges to get the sluggish economy growing, invest in infrastructure and make the UK a "clean energy superpower".
But nothing has really gone wrong in its campaign either.
The party has won the support of large chunks of the business community and endorsements from traditionally conservative newspapers, including the Rupert Murdoch-owned Sun tabloid, which praised Starmer for "dragging his party back to the centre ground of British politics".
But for many voters, the lack of trust applies not just to the governing party but to politicians in general
Veteran anti-establishment figure Nigel Farage has leaped into that breach with his Reform UK party, grabbing attention with calls to curb immigration.
The centrist Liberal Democrats and environmentalist Green Party also want to sweep up disaffected voters.
with Reuters
