Conservative MP Natalie Elphicke has defected to the Labour Party, saying the Tories “have become a byword for incompetence and division”.
In a statement released just as PMQs was starting, the MP for Dover said the key factors for her decision were housing and border security.
She accused Rishi Sunak of “broken promises” and abandoning key pledges.
It is the second defection to Labour for Rishi Sunak in less than two weeks, after Dan Poulter also quit the Tories.
Labour will retain their existing candidate in Dover and Deal at the next general election and Ms Elphicke will stand down, the BBC understands.
The constituency replaces Dover, where Ms Elphicke had a majority of 12,278 at the last election, following boundary changes.
In dramatic scenes, Prime Minister’s Questions began in the Commons with Ms Elphicke crossing the floor to sit behind Labour leader Sir Keir Starmer.
There was bemusement and confusion from many on the Conservative benches.
The news has only been announced at midday and plenty of Tory MPs had not realised that their former colleague was now sitting opposite them – directly behind Sir Keir and so in the camera shot when he is talking – rather than on their side.
Sir Keir welcomed her to the party, asking 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”.
Dover is the arrival point for many people who cross the Channel in small boats.
In her statement, Ms Elphicke said Mr Sunak’s government “is failing to keep our borders safe and secure”, with lives being lost in the Channel and small boat arrivals “at record levels”.
On housing, she said the government was “failing to build the homes we need” and had “betrayed” renters and leaseholders by not delivering on promises to end no fault evictions and abolish ground rents.
“When I was elected in 2019, the Conservative Party occupied the centre ground of British politics,” she said.
“Since then, many things have changed. The elected prime minister [Boris Johnson] was ousted in a coup led by the unelected Rishi Sunak.
“Under Rishi Sunak, the Conservatives have become a byword for incompetence and division. The centre ground has been abandoned and key pledges of the 2019 manifesto have been ditched.”
By contrast she said Labour had “changed out of all recognition” since 2019 and now “occupies the centre ground of British politics”.
She added that the party had a “plan to build the homes we need” and its economic and defences policies “are responsible and can be trusted”.
Ms Elphicke was elected in 2019, taking over the Dover seat which had been held by her disgraced then-husband Charlie Elphicke.
He was jailed for two years in September 2020 for sexually assaulting two women.
It comes after former minister Mr Poulter also defected to Labour, over the Conservative’s record on the NHS.
This is the fourth Conservative defection since 2019.
Lee Anderson, who sat briefly as an independent, joined Reform earlier this year, while Christian Wakeford left the Conservatives for the Labour Party in 2022.
A Conservative Party spokesperson said: “The people of Dover and Deal will be disappointed having felt the impact of illegal immigration.
“They did have an MP who sat with the party fighting to tackle this issue head on, now they have an MP in a party that has worked to block our plans to tackle illegal immigration 139 times.”