Rishi Sunak has been dealt a fresh blow after Tory MP Natalie Elphicke defected to Labour, hitting out at his “tired and chaotic government”.

The MP for Dover said the Prime Minister had failed to deliver on his promise to stop the boats and also accused him of deserting the political centre ground.

She crossed the floor in the Commons just moments before Prime Minister’s Questions and sat behind Sir Keir Starmer, who hailed her defection as another indication of how he had changed the Labour Party.

“From small boats to biosecurity, Rishi Sunak’s government is failing to keep our borders safe and secure,” she said.

“Lives are being lost in the English Channel while small boat arrivals are once again at record levels.

“It’s clear they have failed to keep our borders secure and cannot be trusted.”

Ms Elphicke said the “key deciding factors” in her defection were housing issues and “the safety and security of our borders”.

Speaking to reporters in Sir Keir’s parliamentary office after Prime Minister’s Questions, she said: “In 2019, the Conservatives stood on a manifesto that was very much centre ground, but under Rishi Sunak they’ve abandoned the centre ground and broken many election promises.

“Meanwhile, under Keir Starmer, Labour have changed. And I think that change is going to bring a much better future for our country, and that’s why I was so keen to join the Labour Party and play my part in bringing that important future forward.”

A year ago, Ms Elphicke used a newspaper column to claim “not only have Labour got no plan of their own to tackle illegal immigration, they simply do not want to” and said Sir Keir “has pledged to rip up our world-leading partnership to remove illegal migrants to Rwanda”.

Asked about her previous criticism of Labour over immigration policy, she said Mr Sunak “was the man who said he would stop the boats” but so far this year there had been “record numbers of small boats arrivals”.

“So, he’s not stopping the boats and he’s letting the country down.

“Meanwhile under Labour, they are clear that it is important to have defence and they want to make sure that they have good national security. So I think we should have confidence that Labour are the party who will tackle this issue of the small boats crossings.”

Ms Elphicke is standing down at the general election and denied that she had been offered a peerage by Labour.

The MP could take on an unpaid role working on housing policy with Labour, aides suggested.

Sir Keir urged Tory voters to follow Natalie Elphicke to his party and dodged a question about whether Labour was in talks with other Conservative MPs.

Asked if there could be more defections, he said: “I think there are very many Tory voters who genuinely feel that the party that they may have voted for – many, many times in some cases – is no longer the Tory Party that they see.

“And I say to every Tory voter who feels that they want to be part of a national mission to change our country for the better that the project we’ve built here in this changed Labour Party is a project that I hope they would feel they could get behind. “

The defection comes after MP Dan Poulter’s decision to leave the Tories for Labour in April and the dismal local election results for the Conservatives last week.

At Prime Minister’s Questions, Labour leader Sir Keir Starmer asked Mr Sunak “what is the point of this failed Government staggering on” when “the Tory MP for Dover on the front line of small boats crisis says the Prime Minister cannot be trusted with our borders and joins Labour?”

Ms Elphicke was elected as Dover’s Conservative MP in 2019, taking over the seat which had been held by her disgraced then-husband Charlie, who was jailed for two years after being found guilty in 2020 of sexually assaulting two women.

She stood by him during his trial but she said their marriage ended with his conviction.

Government minister Huw Merriman said he is “absolutely staggered” over Ms Elphicke’s “shameless” defection to Labour.

The rail minister told BBC News: “I’m absolutely staggered – I’ve seen some sights in this place, but actually the lack of scruples on this one is a new bar that Natalie has created.”

He added: “She is just being opportunist, I’m afraid to say, and I’m just disappointed for politics that she’s done what she’s done today.”

A Conservative member of Dover District Council described the defection as a “kick in the guts”.

Councillor Stephen Manion, who represents Eastry Rural, said he was “sickened” by Natalie Elphicke’s decision to cross the floor.

Tory former minister Stephen Hammond said “one of the reasons why I’m so surprised is that she’s always been on the right of the Conservative Party”.

He told Sky News: “If there’s been someone who has done as much as anyone to drag my party away from the centre ground of British politics in the last five years, it’s been Natalie.

“When you defect you are disliked by your old party and distrusted by your new, and she’ll find that out fairly quickly.”

Downing Street insisted Mr Sunak would not change course over his immigration policies in the wake of the Dover MP’s defection.

The Prime Minister’s press secretary brushed off questions on whether Mr Sunak is concerned about the second defection in recent weeks, saying he is focused on “the priorities of the British people”.

The press secretary said she hoped Ms Elphicke would explain why she changed her mind after being “very forthright in attacking ‘Sir Softie’ Starmer” and “Labour’s open-borders policy”.

SNP Westminster leader Stephen Flynn highlighted a social media post from Ms Elphicke urging Tory members to vote for Liz Truss during the 2022 leadership campaign.

“Meet Natalie, the new Labour MP,” he said. “What does this say to their values?”