Video report by ITV News Political Correspondent Romilly Weeks
Amber Rudd has warned that moderates are being purged from mainstream parties in a move that could cost the Tories a majority at the next election.
In her first appearance since quitting the Conservative Party over the Prime Minister's "purge" of so-called rebels, the former work and pensions secretary said her decision to suddenly quit was influenced by the sacking of 21 Tory rebels, who she described as "good moderate Conservatives".
Speaking on BBC One's The Andrew Marr Show, the Hastings and Rye MP also claimed not enough work is being done to leave the European Union with a deal, and said that despite repeatedly asking, she was only shown a "one-page summary" when asked for evidence of negotiations.
Ms Rudd, who has been replaced by Therese Coffey, also said "80% to 90%" of government time is going into preparing for no-deal.
The 56-year-old added she had taken the added step of quitting the party to "make the point that the Conservative Party should be a moderate party which accepts people with different views on the EU".
But allies of Boris Johnson defended the move, with Foreign Secretary Dominic Raab insisting the former London mayor was "right to restore some discipline".
Mr Johnson last week sacked a slew of senior members of his own party, many of them having served in top Government jobs - including former chancellors Ken Clarke and Philip Hammond, and Winston Churchill's grandson Sir Nicholas Soames - after they supported opposition moves to block a no-deal Brexit.
In her resignation letter, Ms Rudd said the sackings of members of the so-called Rebel Alliance were an "assault on decency and democracy" and acts of "political vandalism".
Ms Rudd said she confronted the Tory leader "several times" in meetings over the past week to raise concerns about the direction of the party but chose to resign after failing to change his mind.
"It's not just 21 individuals, it's a big symbol that the Conservative Party doesn't embrace moderate people," she said.
"Ken Clarke, Philip Hammond, these great Conservatives. I kept on arguing against it, but I wasn't getting anywhere."
Ms Rudd, in criticism of MPs currently occupying Cabinet positions but who had voted against Theresa May's Brexit deal, said Britain would have "left the EU by now" if they had voted for the former prime minister's deal.
"Under the last prime minister we had rebellions over European matters - I think Jacob Rees-Mogg rebelled, I'm told, 100 times, lots of people rebelled on the Withdrawal Agreement," she said.
"I think this is disproportionately unfair to single out this group who have a different view on leaving the European Union."
Health Secretary Matt Hancock reacted to the resignation of Ms Rudd and said he was "sorry that [she] had chosen to resign."
"She's been a brilliant minister, a close colleague of mine for many years and I think we need to stick together," he said.
However Mr Hancock said he wanted to find a way to bring the 21 Conservative rebels who voted against the Government "back into the fold."
Ms Rudd posted her resignation letter on Twitter on Saturday night, explaining: “I cannot stand by as good, loyal moderate Conservatives are expelled.
“I have spoken to the PM and my Association Chairman to explain. I remain committed to the One Nation values that drew me into politics.”
In her resignation letter, she tells the PM: "I joined your Cabinet in good faith; accepting that 'no deal' had to be on the table, because it was the means by which we would have the best chance of achieving a new deal to leave on October 31.
"However, I no longer believe leaving with a deal is the Government's main objective."
In a further stinging attack, she says: "I must also address the assault on decency and democracy that took place last week when you sacked 21 talented, loyal One Nation Conservatives.
"This short-sighted culling of my colleagues has stripped the party of broad-minded and dedicated Conservative MPs I cannot support this act of political vandalism."
Ms Rudd also issued a stark warning to the PM that any lurch to the right would damage his chances of securing a majority at the next election, despite a YouGov poll in The Sunday Times suggesting the Tories had taken a 14-point lead over Labour.
A Number 10 spokesperson expressed the Prime Minister's "disappointment" as Ms Rudd's departure.
He added: "She was a talented welfare minister but all ministers who joined joined Cabinet signed up to leaving the EU on October 31st come what may, delivering on the referendum result as the public were promised.
"That has not changed.”.
Such a high profile resignation from his heart of his Cabinet will surely come as a huge blow to Mr Johnson, who has been fighting a rearguard action since MPs took control of the Commons this week to force him to seek an extension to Brexit on October 31.
ITV News Political Correspondent Romilly Weeks on the impact of the "apocalyptic" resignation for Boris Johnson
Brexit Party leader Nigel Farage has offered Mr Johnson a "non-aggression" pact at the next election in a move the prominent Eurosceptic predicted could secure a 100-seat majority.
"If we become a party which has no place for the type of moderate that I am, then we will not win," said Ms Rudd.
The former home secretary said the issue was not confined to her own party, with Labour's Diana Johnson facing a deselection battle after the former minister became the first MP to be forced to fight to remain a candidate under new watered-down rules agreed by members last year.
"Both parties are having difficulty maintaining their moderate side," she added.
Ms Rudd told the BBC she hoped the Tory whip would be restored to her and the sacked MPs before the next election is called.
The PM is set to ask the Commons on Monday to approve his second request for a snap poll, having failed with a bid to secure an election last week.
Ms Rudd said: "I'm actually not leaving the Conservative Party.
"What I am doing is surrendering the whip alongside my colleagues, the 21 others, in order to stand with them.
"I know I couldn't carry on in the Conservative Party at such a high level and see 21 of my colleagues, who are good, moderate people who also want a deal, excluded from it. I just needed to move and stand by them.
"I hope that we will all be returned before a general election so we can all stand as Conservatives. I am a Conservative."
David Gauke, the former secretary of state for justice and lord chancellor and one of those booted out the party as he voted against the government during the week, tweeted: "I'm sure this has not been an easy decision.
"But it is brave and principled and is all about putting the national interest first."
Ms Rudd, MP for Hastings and Rye, quit as home secretary in April last year amid allegations of inadvertently misleading Parliament over the Windrush scandal.
But she was brought back into the fold in November and despite being a Remainer and a vocal supporter of Mr Johnson's leadership rival Jeremy Hunt, she kept hold of her position when Mr Johnson won.
Ms Rudd was also Women and Equalities Minister. No replacement for this role has yet been announced.
Ms Rudd's resignation came hours after Tory grandee Lord Heseltine said the party was fighting for its very soul under the Johnson regime.
The former deputy prime minister, a committed pro-European, said there was “deep unease” with the way the Prime Minister and his “zealot” advisers were behaving.
It's also another blow for Mr Johnson coming just days after his brother, Jo, also walked out on him and the party.
Justine Greening, one of those to have the whip withdrawn, told Sky News' Sophie Ridge On Sunday that she was told of her dismissal after listening to a voicemail from Chief Whip Mark Spencer after getting off the Tube.
The former education secretary, who represents the Remain-voting constituency of Putney, said: "I think it might have been polite after 14 years to try to call me back perhaps a second time. I think it says an awful lot more about, I guess, the state of the Conservative Party than it does about me.
"I have served my community and my country for 14 years and I think it's a shame that the whip has been withdrawn for me doing the simple thing of representing my community in Parliament."
Shadow attorney general and Labour peer Baroness Chakrabarti branded the PM's decision to "purge" centrist Conservatives from Government as "very un-British".
But Foreign Secretary Dominic Raab stood by the move to oust the rebels.
"I think the Prime Minister was right to restore some discipline and I think he's right to expect it from his top team," he told Sky News.
"It's been a rough week, but the reality is the Prime Minister is sticking to his guns on what he said to get us out of this rut that we're in."