The chief Remoaner, who led the Supreme Court battle to give Parliament a vote on triggering Article 50, criticised the Prime Minister after she failed to join other leaders for an election debate on the BBC.
Instead Theresa May sent Home Secretary Amber Rudd in her place to take on Jeremy Corbyn, Tim Farron and other leaders.
Mrs Miller's seized the opportunity to attack the Prime Minister, implying it does not bode well for her negotiations in Europe over Brexit.
General Election – Gina Miller recreates the Nigel Farage Brexit flotilla pose
I thought it was totally scandalous
Asked about Theresa May's absence at the BBC debate, she said: "I thought it was totally scandalous.
"She's the one who called the election, she should be engaging in debate and going out on the doorstep, talking to ordinary people.
"[She should] not be doing invitation-only events around the country and refusing to turn up and debate other leaders.
"What does that say about her negotiating with 20 other leaders, if she can't even negotiate with domestic leaders?"
Gina Miller pictured overlooking the River Thames with Lib Dem candidate George Turner
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Mrs Miller, who is voting Lib Dem, backed Mr Turner's candidacy
Her outburst came as she joined forces with the Liberal Democrats in a desperate bid to canvas for votes.
In an attempt to copy the Nigel Farage she took part in a Leave-style Brexit flotilla photo opportunity, pictured overlooking the River Thames yesterday afternoon with Lib Dem candidate George Turner.
Mr Turner is hoping to take Vauxhall from Labour candidate Kate Hoey, who has been MP for the area since 1989.
During the height of the EU referendum, campaign Brexiteers Miss Hoey and Nigel Farage were pictured aboard a boat as part of the infamous Brexit flotilla.
Pro-Remain and anti-EU demonstrators took their battle to the waters in the run-up to the referendum last year in a shouty Thames showdown which saw Mr Farage and Miss Hoey take on singer Bob Geldof in a protest about EU fishing policies.
But Mrs Miller and Mr Turner stalled at the first hurdle by posing on a beached boat in Lib Dem colours at the Albert Embankment in Vauxhall.
Around 78% of Vauxhall residents voted to remain in the EU
Mrs Miller attempted mirror a picture taken last year featuring former Ukip leader Nigel Farage
Europhile Mrs Miller told reporters she usually votes Labour but will be voting Lib Dem on June 8.
She has been spearheading a pro-EU campaign dubbed Best for Britain funding candidates “who campaign for a meaningful vote on our future relationship with Europe” in a tactical attempt to stop a Tory landslide.
The Best for Britain campaign, launched in April, aims to combat "apathy" amongst an electorate suffering from voter fatigue, Mrs Miller said.
She backed Mr Turner's candidacy, and said the group will be pushing for "real game-changers, strong candidates who will hold whatever government to account and will fight this extreme Brexit that the Tories are peddling" during the General Election campaign.
Things you may not know about remoaner Gina Miller Thu, March 2, 2017
Gina Miller is leading the challenge that has blocked Theresa May triggering Article 50 without the consent of Parliament
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Gina Miller has won the case to block Theresa May from Article 50
The London investment manager has ruled out standing as an MP herself, choosing to lead the Best for Britain campaign despite polling experts casting doubt over how much influence tactical voting can have.
The Lib Dems hope Mr Turner will overturn Ms Hoey's 12,000 majority in a constituency that saw 78 per cent vote for Britain to remain in the EU.
Mr Turner said: "There [is] no illusion as to what the people of Lambeth feel about Brexit. It was the highest voting Remain constituency in the country.
"We can't be in a situation where their MP so brazenly refuses to listen to the constituency. That is what has happened here."
The Vauxhall Liberal Democrats recently released a campaign leaflet which merged the faces of Miss Hoey and Mr Farage, beside the words "Hoey out! Vauxhall deserves better".