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Ms Miller, who has previously challenged the Government on implementing Brexit without approval from MPs, described the Prime Minister’s decision to prorogue Parliament as “shocking”.
Speaking to BBC News, Ms Miller said the PM was "hijacking the Queen's prerogative power" and using it for "unscrupulous means”.
She added: "I think that is what so shocking about this, is that it’s a very cowardly way of using these powers and constitutional convention.
"Our unwritten constitution is a bit like a gentleman's agreement, and you have to say it's not been used in that manner.”
In 2016, Ms Miller launched a successful legal bid, with judges ruling that MPs would have to vote before the Government could invoke Article 50 to formally start the UK's exit process from the EU.
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A separate bid has been launched by pro-Remain barrister Jo Maugham, director of the Good Law Project, who has filed a motion asking the Scottish Court of Session to suspend the PM request that Parliament be prorogued.
Former Tory prime minister Sir John Major also said on Wednesday he is seeking advice on the legality of Mr Johnson proroguing Parliament.
More than one million people have signed a petition calling on the PM not to suspend Parliament, while thousands of people rallied for hours outside Parliament on Wednesday night, with smaller demonstrations taking place in other towns and cities.
The plan to suspend Parliament was heavily criticised by opposition parties and some Conservative MPs.
Mr Johnson said he wanted to prorogue Parliament in order to bring the current record-breaking session to a close in order to bring forward his Government's new legislative agenda.
But opposition leaders said the Prime Minister is trying to halt their efforts to block a no-deal Brexit.
MPs will return to Parliament on Tuesday, but just over a week later on September 10, at the earliest, Parliament could be prorogued until October 14 ahead of a Queen's Speech.
Mr Johnson's prorogation plan came just a day after opposition leaders struck a deal to try to block a no-deal Brexit through legislative means.
Liberal Democrat leader Jo Swinson told the BBC's Newsnight programme that anti-no-deal MPs could use "arcane and unusual" legislative routes to try to block the UK leaving the EU without a deal.
She added: "There are legislative avenues being explored between cross-party groups of members of Parliament with legal and constitutional experts looking at how this can be done, and different routes.
"We have got a Government that is prepared to take unprecedented routes and so we are looking at options as well that might be arcane or unusual that could be employed."