Dan Poulter, a working medic, has quit Prime Minister Rishi Sunak's party saying it is "failing" on the National Health Service
He will take the Labour whip until the general election, when he will not be running again as the MP for Central Suffolk and North Ipswich, he told the Observer newspaper.
Poulter told the newspaper the Conservative Party "feels like it has become a nationalist party of the right," having seen a drift since former prime minister David Cameron's era.
"The health service has ceased to be an area of priority for the Conservative Party, and that is now showing in the strain on the front line and the deterioration of care for patients," he said.
Labour MP Streeting said on social media site X: "Proud to welcome Dr Dan Poulter MP to the @UKLabour Party.
"As a frontline clinician, he's seen the damage that 14 years of Conservative government have done to our NHS.
"Delighted to have his support and look forward to working with him, especially on mental health reform."
The defection is a significant blow to Sunak less than a week before council and mayoral elections in which the Conservatives are expected to suffer heavy losses.
It marks the first time a Conservative MP has crossed the floor to Labour since Christian Wakeford did so in 2022.
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