The sister of the former foreign secretary and prominent Brexiteer Boris Johnson made her surprise announcement as she appeared at the launch of the party’s election campaign.
Rallying against Brexit, Ms Johnson said she did not want to see it “rubbing out my children’s prospects and chances of living and travelling and working in Europe”.
“These are chances that the politicians who decided to campaign to Leave have enjoyed themselves,” she told the Evening Standard.
“It is simply not fair – and sometimes one has to stand up and be counted. It is now that time for me.”
Ms Johnson – formerly a campaigner for the Liberal Democrats – will represent the party for the South West region and said she had called her brother Boris to inform him of her decision.
The journalist and campaigner continued: “A vote to leave the EU is so important, so life-changing for the next two generations that I am impelled to stand up and be counted for what I believe in, which is that we are far better in Europe.”
“I’m honoured they have picked me to stand in the South West, which is where I call home. I went to school there, my father was born in Penzance and we have family dating back to the 1600s in my local village.”
Ms Johnson joined dozens of candidates on stage in Bristol for the launch of the party’s European election campaign including former BBC journalist Gavin Esler, ex-Tory minister Stephen Dorrell and QC Jessica Simor.
Mr Esler, who is standing as a candidate in London, slammed Brexiteers such as Nigel Farage and Jacob Rees-Mogg as the “posers of Brexit”, who were simply ”selling the same old snake oil”.
He said: “I have never been seriously worried about the future of our country but I am now.
“Our political system is a joke. It is a worldwide joke. They are laughing at us – not with us, at us.”
Interim leader Heidi Allen said 3,700 people had offered to stand in the elections, with candidates from “every corner of the UK” and from “all walks of life”.
“These elections are a chance to send the clearest possible message – we demand a people’s vote and the right to campaign to remain in the European Union.
“We are not afraid to say it as clearly as that.”
The former Tory MP added: “This is no rebel alliance – this is the home of the Remain alliance.”
Ms Allen also suggested that Change UK could back the government in a no-confidence vote to avoid a damaging general election.
Speaking to The Independent after the launch, she said an election was “absolutely the last thing” the country needed during the Brexit chaos.
She also dismissed questions over lack of clarity on what the party stood for aside from its opposition to Brexit.
“Yes there will be things that we will disagree on, for sure,” she said.
“[Ex Labour MP] Chris Leslie jokes that I am more left-wing on welfare than he is.
“But the point is if we can back policy between us and come up with a compromise, isn’t that a great sign that we can sell that to the British public as they are broadly in the centre ground?”