Andrew Lewin is the new MP for Welwyn Hatfield after Labour gained the constituency from the Conservatives, ending Grant Shapps' 19 years in the seat.
In a closely fought race, Mr Lewin won 19,877 votes compared to 16,078 for Mr Shapps, with a loud cheer going up from Labour members at Campus West as the result was announced just after 3am.
Mr Lewin said he was "honoured" to be the new MP for Welwyn Hatfield, telling reporter Dan Mountney: "I am delighted with the result and it’s such a privilege to now be the Labour MP for the place I call I home.
"I was born in QEII, I live in this community with my family and this is an honour. I know there is a huge amount of work to do now."
Throughout his campaign, Mr Lewin reaffirmed that despite what the polls were predicting, he believed it would be a close race between himself and Mr Shapps.
He described his bid to become MP as a "fight", adding: "I knew this was going to be close, and it was close the whole way through.
"We had to fight for this election as hard as we could, but also with a positive message. I’m proud of the campaign we ran, and people really responded to it. I had countless conversations where people were positive about the changes we were going to make."
During his speech, Mr Lewin was quick to thank Mr Shapps for his service during his 19 years as MP for Welwyn Hatfield, with the Conservative candidate responding in kind as he congratulated his Labour opponent.
The defence secretary was also keen to thank his constituents for their support during his near two decades in the seat, saying it was a "privilege" to serve Welwyn Hatfield.
Arriving just moments before the declaration of the result, a clearly emotional Mr Shapps gave his speech and then departed Campus West without giving an interview to the press.
Elsewhere, Reform UK's Jack Aaron finished in third with 6,397 votes, and he already has one eye on the next general election, expressing his belief that he can win the seat in 2029.
"As people have been saying, Reform UK has been building the wheelbarrow as we push it along," he told the Welwyn Hatfield Times.
"I went out around a Hatfield and every other person was suggesting I had their vote. My view is that I could win in 2029."
The Liberal Democrat's and John Munro finished on 3,117, with the Green Party's Sarah Butcher getting 2,986.
The turnout of 64.6 per cent was the second lowest in the history of Welwyn Hatfield constituency, only beaten by the 63.9 per cent turnout in 2001, when Labour's Melanie Johnson won the seat.
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