Mark Logan, who stood down as a Conservative MP when parliament dissolved on Thursday, is backing Labour at July's General Election.
The Bolton North East MP said the Tory party is "unrecognisable" from the one he joined a decade ago, adding that Labour now offers "centrist politics".
The Brexit-supporting former businessman had just a 378 majority in his seat in 2019, one of the most marginal in history.
Asked on whether he could run for Labour at a future election, he told the BBC: "I wouldn't rule out coming back into public life in the future but this is me definitely stepping down in this Parliament.”
He continued: "The time has come to bring back optimism into British public life.
"For my constituents and for the country, it's right that we get some stability back into the UK, we get optimism, we get new and fresh ideas."
Mr Logan broke ranks from the Tory Party in February this year when he called for an immediate ceasefire in the Gaza conflict.
He said Sir Keir Starmer is "best placed" to deal with the conflict.
Tories hit back
In response, a Conservative Party spokesman said: "Much of the association wanted him deselected for inactivity and his failure to be present in Bolton for extended periods of time. And he's been asking No 10 for a peerage".
Another said: “It’s notable that Mark Logan has defected to a party he can’t even name a single policy of.
"We wish Mark Logan well with the Labour Party - a party that has no plan for the country and would take us back to square one."
Two other former Tory MPs, Dan Poulter and Natalie Elphicke, joined Labour last month. They each sat as Labour MPs prior to parliament being dissolved.