The Liberal Democrat promise to revoke Article 50 is "arrogant" and "self-indulgent", former Green Party co-leader Caroline Lucas has told ITV News.
Ms Lucas insisted her criticism wasn't based on fear of losing votes to Jo Swinson's party, which is pitching itself as the natural home for Remain supporters at the next election.
Asked whether it made it harder to form a Remain election pact with the Lib Dems - which would see the parties agree not to compete for particular seats - she demanded a greater commitment from her closest political rivals.
Ms Lucas told Brand the Lib Dems and Labour both needed to show a genuine intent "to help some Green Party voices to get into Parliament" after she felt let down by her party's sacrifices in the 2017 election.
The full podcast interview will be released on Thursday.
Asked by Brand if the Greens would also revoke Article 50, Ms Lucas said: "Absolutely not. I am deeply concerned about this policy of revocation as a first resort rather than as the very, very last as we're just about to tip out of the EU.
"It imperils democracy ... I think it's arrogant, I think it's self-indulgent."
She added: "Obviously (the) Lib Dems have done some polling that tells them that this is the right policy in terms of some short-term votes but in terms of the long-term harm that I think this will be, that will be done to our country through doing this.
"I think that it's incredibly dangerous. I'm deeply worried by it."
Ms Lucas, who stepped down from the party co-leadership last year, runs a project 'Dear Leavers' to speak to Brexit voters across the country in the hope of finding common ground.
In the podcast the MP went on to add: "We're already such a divided country.
"By saying to 17.4 million people 'we're not listening to you and we don't care why you voted leave and we're just going to put the clock back to the 22nd June 2016', I think that is the biggest kick in the teeth that you could possibly deliver."
The Greens will push at the next election for a People's Vote to rerun the EU referendum and would campaign in favour of Remain again.
Asked if she would work with other parties to bolster the chances of Remain-supporting MPs being elected, Ms Lucas accepted it would have to be a two-way street in agreeing to stand down candidates.
"Essentially we made a big gesture in 2017 (not to stand in certain constituencies) and we paid a high price for it, quite frankly," she said.
"We're not expecting to win as many seats as those other much bigger parties but there needs to be ... an element of reciprocity in this for sure."
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