Mike Amesbury has announced he will stand down as an MP after he was convicted of punching a man in the street.
A by-election will now be triggered in his seat of Runcorn and Helsby, where constituents will vote to elect a new MP.
Amesbury, who was suspended from the Labour Party, was jailed on 24 February for 10 weeks after he pleaded guilty to assault by beating of 45-year-old Paul Fellows in Main Street, Frodsham, Cheshire, in the early hours of 26 October.
However, following an appeal, his sentence was suspended for two years, so he does not have to serve it in prison.
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Amesbury, 55, said in a statement he has decided he “must step down” as an MP, and will do so “in the coming days”.
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His resignation will trigger a by-election in his Cheshire seat – the first of Sir Keir Starmer’s Labour government.
Reform UK started targeting the seat even before Amesbury’s sentencing and remains the bookies’ favourite, with Labour coming in second.
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Amesbury said his actions were “wrong” and that he made a “serious mistake” for which he is “profoundly sorry to everyone I have let down”.
He said there was no excuse for his “moment of madness” but said the exposure from being an MP “has brought about some distressing experiences”, including a stalker who, despite a restraining order, attended his plea hearing in January.
“It can put you on edge in certain circumstances,” he said.
He told the BBC he regrets the attack “every moment, every day” and said he would have tried to remain an MP if he had been given a lighter community sentence.
Parliamentary rules state any prison sentence, even suspended, given to an MP triggers a recall petition – but only after any appeals have been exhausted.
A by-election is then called if 10% of constituents vote to remove their MP. However, as Amesbury will voluntarily step down, no recall petition will take place, therefore automatically triggering a by-election.
A Reform UK spokesman said: “We stand ready to contest this by-election, it’s one of Labour’s safest seats but we will give it our all.
“Labour have failed Runcorn, Helsby and country since taking office. Runcorn and Helsby needs Reform.”
Political correspondent
Since Mike Amesbury was convicted of assault, it seemed almost possible he would save his political career.
Today, we finally got the confirmation that he won’t, as he announced he is stepping down and triggering a by-election.
It’s the first of Sir Keir Starmer’s premiership and will be a big test for the prime minister in a seat where Reform are second.
Losing it would be an unmitigated disaster, given the 14,000 majority achieved last time around.
But even if Labour does hold on, any significant shifts to Reform will be a boost for Nigel Farage’s party.
The most significant factor to keep an eye on will be whether their impressive polling gains can be converted into votes at the ballot box.
Knowing the damage that will do to morale on the government benches, Labour HQ will fight hard.
On the other side, well-resourced Reform will also throw everything at it, aware that it could be a pivotal moment in British politics.
Amesbury has continued to take his £91,000 salary after he was sentenced, including when he spent three nights in prison before his appeal was successful. The rules state MPs receive their salaries only until they are no longer MPs.
The MP said he carried out casework while behind bars as his office manager forwarded on emails.
“Life doesn’t stop as an MP,” he told the BBC.
In a statement, he said he now has to find a job “but with a criminal conviction and a tarnished reputation”, as he said he hopes to learn from the experience and rebuild over time.
“I have nobody to blame but myself,” he added.
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Labour suspended Amesbury from the party shortly after the incident, so he has been sitting as an independent MP in the Commons.
The party said he would not be readmitted to Labour and had called for a by-election, saying Amesbury’s constituents “deserved better” after his “completely unacceptable actions”.
At his appeal, Amesbury was also ordered to carry out 200 hours of unpaid work, undertake a 120-day alcohol monitoring requirement, go on an anger management course and carry out 20 days of rehabilitation work.
