MP Natalie Elphicke has defected from the Tories to Labour, in another humiliating blow for Rishi Sunak following his hammering at the local elections.
It came just minutes before the prime minister faced Keir Starmer at PMQs on Wednesday.
The now Labour MP for Dover accused Sunak of “failing to keep our borders safe” and of “incompetence and division”.
It is a particularly stinging attack on Sunak, given he has made stopping small boat crossings from France to Kent central to his election campaign.
In a statement on Wednesday, Elphicke said: “Rishi Sunak’s government is failing to keep our borders safe and secure.
“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.”
HuffPost UK revealed on Sunday that two Tory MPs were in talks about defecting before the general election.
Elphicke, who was elected 2019, also accused Sunak having “ousted” Boris Johnson as prime minister in a “coup”.
“Under Rishi Sunak, the Conservatives have become a byword for incompetence and division,” she said.
Starmer hailed the defection as Labour renewed calls for a general election to be called.
“How many more times does the public and his own MPs need to reject him before he takes the hint?” he said.
Two MPs elected as Tories in 2019 - Christian Wakeford and Dan Poulter - have already crossed the floor in this parliament.
Poulter stunned Westminster last weekend when he left the Tories for Labour, blaming the government’s handling of the NHS.
Wakeford, the MP for Bury South, dramatically defected to Labour in January, 2022, over the partygate scandal.
The Conservatives lost nearly 500 council seats at the local elections last week, as well as all but one mayoral race.
Pollsters continue to predict that the Tories are heading for a heavy defeat at the general election.