Tory MP Natalie Elphicke has defected to Labour, hitting out at the "broken promises of Rishi Sunak's tired and chaotic government".
She was spotted sitting on the Labour benches ahead of today's PMQs.
Elphicke, the MP for Dover, has been vocal in her campaign to crack down on migration.
In a brutal statement, she accused Sunak of failing to "keep our borders safe and secure", as well as "failing to build the homes we need".
The latest defection means that the Government's majority has more than halved since the general election, dropping from 80 to 38 seats.
She said: "Today I announce that I have decided to join the Labour Party and that I will sit in Parliament as a Labour MP.
"When I was elected in 2019, the Conservative Party occupied the centre ground of British politics. The party was about building the future and making the most of the opportunities that lay ahead for our country.
"Since then, many things have changed. The elected Prime Minister 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.
"Meanwhile the Labour Party has changed out of all recognition. Since 2019, it has moved on from Jeremy Corbyn and now, under Keir Starmer, occupies the centre ground of British politics. It has accepted Brexit and its economic policies and defence policies are responsible and can be trusted.
"Most significantly for me, the modern Labour Party looks to the future – to building a Britain of hope, optimism, opportunity and fairness. A Britain everyone can be part of."
Elphicke added: "We need to move on from the broken promises of Rishi Sunak’s tired and chaotic Government. Britain needs a Government that will build a future of hope, optimism, opportunity and fairness.
"A Britain everyone can be part of, that will make the most of the opportunities that lie ahead. That’s why it’s time for change. Time for a Labour Government led by Keir Starmer. The General Election cannot come soon enough."
Starmer welcomed Elphicke to his party at PMQs, asking 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”.
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Elphicke, the MP for Dover, has been vocal in her campaign to crack down on migration.
PA
The Labour leader also asked: “How many more times do the public and his own MPs need to reject him before he takes the hint?”
Sunak responded: “This time last year I reminded him of some advice actually form his own mentor Tony Blair who had said at the time he can be as cocky as he likes about local elections but come a general election it is policy that counts.
“One year on from that advice, what has he managed? £28bn of tax rises, 70 new business regulations, 30 U-turns and a deputy leader under a police investigation.”
Responding to Elphicke's defection, 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 had an MP in a party that has worked to block our plans to tackle illegal immigration 139 times.
"We wish Natalie well as she now has to support Labour's amnesty for illegal immigrants and one that directly opposes her own views - it was only last year that she penned an article titled 'Don't trust Labour on immigration they really want open borders'."