The poll showed Labour would win 410 seats in the 650-seat parliament, ending 14 years of Conservative-led government.
Sunak’s party was forecast to only take 131 seats, down from 346 when parliament was dissolved, as voters punish the Conservatives for a cost-of-living crisis and years of instability and infighting that has seen five prime ministers since 2016.
In the last six national elections, only one exit poll has got the outcome wrong: In 2015 the poll predicted a hung parliament when in fact the Conservatives won a majority. Official results will follow over the next few hours.
Sunak stunned Westminster and many in his own party by calling the election earlier than he needed to in May with the Conservatives trailing Labour by about 20 points in opinion polls.
He had hoped that the gap would narrow as had traditionally been the case in British elections, but the deficit has failed to budge in a fairly disastrous campaign.
It started badly with Sunak getting drenched by rain outside Downing Street as he announced the vote, before aides and Conservative candidates became caught up in a gambling scandal over suspicious bets placed on the date of the election.
Sunak’s early departure from D-Day commemorative events in France to do a TV interview angered veterans, and even those within his own party said it raised questions about his political acumen.
While polls have suggested that there is no great enthusiasm for Labour leader Starmer, his simple message that it was time for change appears to have resonated with voters.
Unlike in France where Marine Le Pen’s far-right National Rally party made historic gains in an election last Sunday, the disenchanted British public appears to have instead moved to the center-left.