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Anti-Brexit campaigner Gina Miller says she will take the government to court if the next PM tries to shut down Parliament to push through no deal.
Ms Miller, who won a legal battle against ministers over Article 50, said the step would be “an abuse” of powers.
She told Sky News she wanted to “defend Parliamentary sovereignty”, not stop Brexit.
Brexiteer MP Priti Patel said it was “not acceptable” to use the courts to try to tie the hands of MPs.
Most MPs are against leaving the EU without a deal and could try to stop it from happening.
Shutting down – or proroguing – Parliament could potentially provide a way for a prime minister to force through a no-deal Brexit against MPs’ wishes.
With Parliament not sitting, MPs would not be able to block it – for example, by holding a vote of no confidence in the prime minister.
Boris Johnson, Conservative leadership candidate and favourite to win the race to be prime minister, has vowed to leave the EU by the current deadline of 31 October with or without a deal.
He has refused to rule out proroguing Parliament in order to achieve that – his rival, Jeremy Hunt, has insisted he would not take that step.
In a no-deal scenario, the UK would leave the EU with no agreement about the “divorce” process, immediately exiting the single market and customs union – arrangements designed to facilitate trade.
Opponents of no deal say it would damage the economy and lead to border posts between Northern Ireland and the Republic of Ireland, but politicians in favour of no deal insist disruption could be quickly overcome.
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Media captionWhat does proroguing Parliament mean?
Ms Miller and her legal team sent a letter to Mr Johnson this week warning him about their plans for legal action in the event he tries to prorogue Parliament.
She told Sky: “I have never been a fan of Brexit… but that is completely separate, completely different, from defending the central pillar of our constitution.”
She said it would be an abuse of power for a PM “through desperation” to “try to limit the voice of the MPs we all elect”.
“We think that it’s beyond the prime minister’s powers because parliamentary sovereignty is actually the jewel in the constitutional crown.”
Ms Patel, a supporter of Mr Johnson, told Sky: “I’m not surprised [at Ms Miller’s remarks] because there seems to be a relentless movement to delay Brexit,