Liberal Democrat MP Heidi Allen has said she will not seek re-election in the looming poll as she is “exhausted” by the “nastiness and intimidation” dominating politics.

The former Conservative MP said the public scrutiny she had endured was “utterly dehumanising” and described receiving threats, abuse and being forced to install panic alarms at her home.

Ms Allen, who backs a Final Say referendum, said Brexit had “broken our politics” and the parliament had become a “purgatory” where nothing could be achieved.

Her announcement came as Sam Gyimah, another former Tory MP, was selected to fight a knife-edge contest in Kensington, where Labour’s Emma Dent Coad holds a wafer-thin majority.

Boris Johnson is on course for an early general election on 12 December, after a one-line bill to grant a snap poll passed its Commons stages in a single day.

Ms Allen announced she “will not re-stand when the next general election comes” in an open letter to her constituents on Tuesday.

She said: “But for the last 18 months or so, the Brexit impasse has made business as usual impossible.

“Brexit has broken our politics and it is my firm belief that only a confirmatory public vote will bring an end to this sorry chapter and bring healing and light at the end of the tunnel.

“And while parliament has been in purgatory, we have legislated for almost nothing, changed almost nothing and improved almost nothing. I became an MP because I wanted to make a difference, but while Brexit continues to captivate parliament, that just hasn’t been possible.”

She added: “But more than all of that, I am exhausted by the invasion into my privacy and the nastiness and intimidation that has become commonplace.

“Nobody in any job should have to put up with threats, aggressive emails, being shouted at in the street, sworn at on social media, nor have to install panic alarms at home.

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“Of course public scrutiny is to be expected, but lines are all too regularly crossed and the effect is utterly dehumanising.”

Ms Allen was one of rebel MPs who joined the Independent Group in February, after resigning the Conservative whip in protest at the government’s handling of Brexit.

She became leader of the group of ex-Labour and Tory MPs, later renamed Change UK, before later joining the Liberal Democrats.

Her South Cambridgeshire constituency is a plum seat for the Tories, where Ms Allen held a majority of nearly 16,000 in 2017.

Meanwhile, Mr Gyimah announced he would give up his East Surrey seat to battle it out in Remain-backing Kensington in the looming poll.

The former minister was one of the 21 Tory rebels sacked by Mr Johnson for supporting backbench legislation to prevent a no-deal Brexit. He joined the Liberal Democrats last month.

Labour MP Owen Smith, a one-time leadership contender, also announced he would also stand down after MPs approved the bill for an election on 12 December.

He said: "For political and personal reasons, I have written tonight to Jeremy Corbyn informing him of my decision not to stand at the coming general election.