Amber Rudd has resigned from the cabinet and quit the Conservative party, she said in a statement.

The Hastings and Rye MP quit her post as work and pensions secretary and said she was relinquishing the Tory whip (which means leaving the party) after the Prime Minister sacked 21 rebels this week.

Boris Johnson expelled (“removed the whip”) two former chancellors and Winston Churchill’s grandson from the Conservative party after they voted to give Opposition MPs control of the order paper and start the process of blocking a no-deal Brexit.

Rudd tweeted: “I have resigned from Cabinet and surrendered the Conservative Whip.

“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.”

The former home secretary, who stepped down following the Windrush scandal, was dogged by questions throughout the Tory leadership contest about whether she could serve in Johnson’s cabinet if he won the race, given his strategy would involve keeping no-deal on the table during further negotiations with Brussels.

She accepted the offer of continuing in her job as work and pensions secretary when Johnson formed his cabinet in July.

But in her letter of resignation, the now independent MP said that while she had accepted the need to keep no-deal as an option, she said she “no longer believed leaving with a deal is the government’s main objective”.

Issuing forthright criticism of Johnson, she called his decision to sack Tory rebels – such as ex-chancellor Philip Hammond, Churchill’s grandson Sir Nicholas Soames and Ken Clarke, the longest serving MP in the commons – an “assault on decency and democracy”.

Rudd, who was also minister for disabled people, added: “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.

“Therefore, it is with regret that I am also surrendering the Conservative whip.”

Rudd has represented her constituency since 2010 and has one of the smallest majorities in the country, with only 346 votes separating her from her Labour rival in 2017.

Conservatives called Rudd’s decision to leave her post and party “brave”.

Health Secretary Matt Hancock tweeted: “So sorry to see Amber resign – a first-rate minister, genuinely wonderful person, and someone I’m proud to call my friend.”

David Gauke, ex-justice secretary and one of the Tories sacked this week, said: “Amber Rudd has been extraordinarily brave. But her concerns about how the Government is behaving reflect the views of many of my (former) colleagues.

“One way or another, it is time for them to act.”

Former Conservative MP and minister Nick Boles – who quit the party in April after his soft Brexit plan failed – tweeted: “Everyone has a point beyond which they cannot be pushed.

“Amber Rudd has reached hers. How much more of the party he inherited will Johnson destroy before he has second thoughts or is stopped by his Cabinet colleagues?”

Responding to Rudd’s resignation, Labour MP Dawn Butler wrote: This is to be admired. We all know Boris is making no effort to negotiate But it also means that since 2010 there has been 8 ministers for women & equalities.

“I’m going to have face another minister at the dispatch box. Boris and the conservatives just don’t care about women”.

Leader of The Independent Group for Change, Anna Soubry, tweeted Rudd’s resignation letter alongside the words: “At last. #respect.”