Sir Keir Starmer has insisted he took "decisive action" over comments made by Rochdale by-election candidate Azhar Ali about Israel and Jewish people.
Labour has been criticised for initially standing by Mr Ali after it emerged he claimed Israel had "allowed" the 7 October attacks by Hamas.
On Monday the party said it was withdrawing support for him after further remarks came to light.
The Labour leader said this was a "tough" but "necessary" decision.
Speaking on a visit to Wellingborough, Sir Keir said: "Further information came to light yesterday calling for decisive action, so I took decisive action.
"It is a huge thing to withdraw support for a Labour candidate during the course of a by-election."
He added: "But when I say the Labour Party has changed under my leadership I mean it."
Mr Ali will still be listed as the Labour candidate on the ballot paper at the by-election on 29 February, as under electoral law it is too late to remove him.
However, it is understood he has been suspended from the party pending an investigation.