Lee Anderson has left the Conservatives for the Reform Party and will sit as their sole MP.
The former Tory deputy party chairman was suspended over his claim that Islamists had control of London mayor Sadiq Khan.
Mr Anderson is a former Labour councillor who worked for ex-MP Gloria de Piero, who was elected as a Conservative MP in 2019.
With a reference to former Tory MP Mark Reckless, who defected to UKIP, he told the BBC political editor Chris Mason it would be “reckless” to call a by-election when a general election was looming.
Ex-Tory MP Lee Anderson defects to Reform
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