Keir Starmer has challenged hard-left Labour MPs who oppose his Government plans to either support him or leave the party.
The Equality and Human Rights Commission is set to remove Labour from special measures for antisemitism on Wednesday morning.
More than two years ago, a report identified “serious failings” under Jeremy Corbyn’s leadership.
Writing for The Times, the Labour leader said the changes he has made to the party are “permanent, fundamental, irrevocable”.
He went on to say that the party was unrecognisable from when he took over in 2019.
“There are those who don’t like that change, who still refuse to see the reality of what happened under the previous leadership,” he continued.
“To them I say in all candour: we are never going back. Nobody is forcing you to stay if you don’t like it.”
When Mr. Corbyn was Labour leader, Sir Keir served as shadow Brexit secretary.
Sir Keir, on the other hand, has sought to distance himself from his predecessor since taking over the party leadership.