Former Conservative MP Dame Andrea Jenkyns has joined Reform UK and will stand as the party's candidate to be Mayor of Greater Lincolnshire in May 2025.
Dame Andrea, who lost her seat in the July general election, said she had "fought to the bitter end" but the Conservative Party was now "beyond salvage".
She said Reform UK were "patriotic bravehearts" who would "deliver the fresh start this country so desperately needs".
It came as the Reform UK's leader Nigel Farage said his party's membership had exceeded 100,000.
He said his party were now focused on the English county council elections in May next year, including plans to stand 2,200 candidates.
He added that Reform would need to win "hundreds of seats" at the elections to be on course for what they hoped to achieve in 2025.
Reform UK, originally called the Brexit Party when it was set up in 2018, had a successful general election.
Although the party only won five parliamentary seats, overall it secured more than four million votes, coming third behind Labour and the Conservatives.
Asked how long she had been thinking about leaving the Conservatives, Dame Andrea said she had "always respected" Farage and had felt "politically aligned" with the party.
"I was tempted before the general election, but I am a loyal person to a party.
"I might not be loyal to prime ministers, as we've seen in the past, but I'm loyal to parties, and I believed, as I said, in going down with that ship fighting.
"I was elected as a Conservative, and I got knocked out as a Conservative, but I feel, unfortunately, the party has become tired."
Dame Andrea had long made clear her sympathies with Reform UK, previously urging the Conservatives to unite with the party in order to prevent a Labour "supermajority".
She also included a picture of herself with Farage on her campaign leaflet in the general election.
Last year, she publicly called on Rishi Sunak, her then leader, to step down as prime minister.
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