Because nominations for the July 4 election have closed, Mr Booth-Isherwood will still appear on the ballot paper.
He said: “Over the past few weeks, I have been increasingly disillusioned with the behaviour and conduct of Reform.
“Whilst I have campaigned alongside many decent, honest and hard-working people during the course of the general election campaign in Erewash, the reports of widespread racism and sexism in Reform have made clear that there is a significant moral issue within certain elements of the party.
“The failure of the party’s leadership to not only take this matter seriously, but also to fundamentally address it, has made clear to me that this is no longer a party I want to be associated with. As a result, I am announcing my endorsement of the Conservative Party candidate, Maggie Throup, for Erewash. Only she can stop Labour.”
Mr Booth-Isherwood had previously said he was disillusioned with the Conservatives and that Reform “better represents the values I wish to campaign on”.
A Conservative Party spokesman said: “If just 130,000 voters like him, currently considering a vote for Reform or Liberal Democrat, voted Conservative it would be enough to stop Labour’s super-majority, which means higher taxes and an illegal immigration amnesty.
“There is no doubt only a vote for the Conservatives can prevent an unaccountable majority for Labour and the ruin this would bring for the country.”
It came as Mr Farage dismissed claims that “Russian-inspired” bots were interfering in the election as “cobblers”, having been targeted by a Vladmir Putin-themed stunt at a campaign rally on Saturday.
