Former cabinet minister Sir Robert Buckland has lost his seat as Labour made its first gain of the night.
The ex-justice and Wales secretary had represented Swindon South since 2010, but will now be replaced by the former Labour MP Heidi Alexander, who beat him by more than 12,000 votes.
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Five other seats have been announced so far, all being held by Labour in the North East of England.
But in the first blows for the Conservative Party, Reform UK came second in all of those five seats, pushing the Tories into third place.
Posting on X, Reform leader Nigel Farage said the results were "almost unbelievable", adding: "And what does it mean? It means we're going to win seats. Many, many seats, I think right now, across the country."
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The Liberal Democrats have also declared victory in 12 seats where results have yet to be announced - all of which were Conservative ahead of the election.
Commenting on the list, which includes Torbay, Eastleigh and Wimbledon, a party source said: "From the West Country to Greater Manchester, the map is being painted gold as Liberal Democrats sweep to victory in the Conservative Party's former heartlands."
It comes as Labour look set for a landslide victory after the exit poll projected Sir Keir Starmer's party would win 410 seats overnight - with an overall majority of 170 in the Commons.
That compares to a prediction of just 131 seats for Rishi Sunak's Conservatives - which would be the lowest seat tally in the party's history - along with the expected loss of some big beasts, including Chancellor Jeremy Hunt.
In the first result of the night, shadow education secretary Bridget Phillipson held Houghton and Sunderland South and said: "Tonight the British people have spoken and if the exit poll this evening is again a guide to results across our country - as it so often is - then after 14 years the British people have chosen change.
"They have chosen Labour and they have chosen the leadership of Keir Starmer. Today our country with its proud history has chosen a brighter future."
Labour's deputy leader - and possibly the next deputy prime minister - Angela Rayner told Sky News that Sir Keir had "done a tremendous job in transforming the Labour Party and putting forward a programme for government that the country can get behind".
Meanwhile, a Conservative Party spokesperson said on Thursday night: "If these results are correct it is clear that Starmer and Angela Rayner will be in Downing Street tomorrow - that means your taxes will rise and our country will be less secure."
The Liberal Democrats are also looking likely to have a strong night, going from 11 seats to 61 in the exit poll, while the SNP could fall to as few as 10 seats - swapping with the Lib Dems as the third largest party.
Reform's first real electoral test could see them win as many as 13 seats, according to the poll - with Mr Farage almost certain to take Clacton.
And the Green Party could gain an extra seat, taking their total to two.
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The results have started coming in after a six-week campaign, launched by Mr Sunak in the pouring rain in Downing Street, which has seen Labour dominate in the polls throughout.
He has faced a number of mishaps throughout the campaign, from his decision to leave D-Day early to his party becoming embroiled in a betting scandal.
But the prime minister insisted he would "fight" to prevent a Labour "supermajority".
Both main parties are playing down the early projections ahead of the final results being announced.
But Labour's shadow environment secretary Steve Reed told Sky News the "big, solid wins" for Labour in the first results "were starting to authenticate the direction of travel that we saw in that exit poll".
Former minister and Tory peer Lord Jo Johnson also told Sky that the election "raised really important issues as to whether it's a sensible thing for the Conservatives to try and be Reform-lite and expect that to be a winning political strategy - it doesn't look to be on the evidence of what we're seeing today."
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