Keir Starmer has suggested that Jeremy Corbyn is unlikely to regain the whip following his latest criticism of Nato.
The former leader has not been allowed to sit as a Labour MP since his comments in the wake of a damning report by the Equalities and Human Rights Commission (EHRC) into anti-Semitism in the party.
The Islington North MP later sought to clarify his remarks and his party membership was restored but he has not been readmitted to the parliamentary party.
Mr Starmer was grilled on his successor's future after Mr Corbyn said last week that he hoped military alliances like Nato would be disbanded.
Mr Starmer told BBC's Sunday Morning: "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 Labour 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 Mr Corbyn returning to the fold, Mr Starmer said: "I'm very clear on my positions on those two issues, very clear."
Last week, Mr Corbyn, who is a long-standing Nato critic, told Times Radio: "I would want to see a world where we start to ultimately disband all military alliances.
"The issue has to be what's the best way of bringing about peace in the future? Is it by more alliances? Is it by more military build-up?
"Or is it by stopping the war in Ukraine and the other wars... that are going on at the present time, which are also killing a very large number of people?
"And ask yourself the question, do military alliances bring peace? Or do they actually encourage each other and build up to a greater danger?
"I don't blame Nato for the fact that Russia has invaded Ukraine, what I say is look at the thing historically, and look at the process that could happen at the end of the Ukraine war."
It comes after Mr Starmer warned a group of backbench MPs they could be kicked out of the party for signing a letter slamming Nato's "eastward expansion".
Mr Corbyn, who has been an MP for nearly 40 years, recently urged the Labour leader to reinstate him.
“I think it’s wrong, it’s unfair, and I should be reinstated. And indeed, an awful lot of people in Islington think exactly the same,” he told the Evening Standard.
Asked if he would stand as an independent MP, he said: "Let’s deal with that bridge when we get to it."
