Keir Starmer said it was “difficult” to see how Jeremy Corbyn could have the Labor whip restored following his comments on Nato.
Last week Corbyn suggested military alliances like Nato could build up “greater danger” in the world and should ultimately be disbanded.
The former Labor leader was stripped of the whip in 2020 over his response to the equities watchdog’s report into anti-Semitism in the party.
He was reinstated as a Labor member after a suspension, but Starmer has refused to readmit him to the parliamentary party.
Corbyn’s allies in the party have repeatedly demanded that their ally be allowed to sit as a Labor MP again so he can stand for the party at the next election.
But asked on the BBC’s Sunday Morning program whether Corbyn could be restored as a Labor MP following the comments, said: “It is very difficult to see how that situation can now be resolved.
He lost the whip because of his response to the Equalities and Human Rights Commission in relation to anti-Semitism but I made it very clear, the first thing I said as party leader was that I was going to tear out anti-Semitism by its roots in our party.
“I’ve also made it clear that our position in the Labor Party is not to accept the false equivalence between Russian aggression and the acts of Nato.”
Told that it sounded as if he was against Corbyn returning as a Labor MP, Starmer replied: “I’m very clear on my positions on those two issues, very clear.”
Corbyn, the MP for Islington North and a long-standing critic of Nato, told Times Radio last week that he did not blame Nato for Vladimir Putin’s invasion of Ukraine but questioned: “Do military alliances bring peace?”
He acknowledged the transatlantic alliance was not going to be scrapped immediately but added that people should “look at the process that could happen at the end of the Ukraine war”.
Corbyn has described the choice to remove the Labor whip from him as a “totally unjustified decision”.
Asked if he could start his new own political party, he said: “I don’t know what the future is going to bring.”
Last week Boris Johnson accused Starmer of being “a Corbynista in a smart Islington suit”.
But Starmer responded: “I think you’ll find Mr Corbyn doesn’t have the whip.”