Labour's deputy leader Tom Watson says Jeremy Corbyn was right to express regret for sending an apparently supportive message to the creator of an allegedly anti-Semitic mural.
Jeremy Corbyn faced criticism over his initial response to a Facebook post by street artist Mear One in 2012.
He later called the mural "deeply disturbing" and backed its removal.
Mr Watson also apologised personally for any hurt that had been caused by his leader's comments.
What caused the row?
In October 2012, Mear One posted a picture of his mural in east London called "Freedom of Humanity" on Facebook, with the words: "Tomorrow they want to buff my mural. Freedom of expression. London calling. Public Art."
Mr Corbyn replied: "Why? You are in good company. Rockerfeller destroyed Diego [Rivera's] mural because it includes a picture of Lenin."
More recently, Labour MP Luciana Berger sought clarification from the leader's office on the 2012 comments - made three years before Mr Corbyn became the party's leader.
Mr Corbyn said: "I sincerely regret that I did not look more closely at the image I was commenting on, the contents of which are deeply disturbing and anti-Semitic.
"I am opposed to the production of anti-Semitic material of any kind, and the defence of free speech cannot be used as a justification for the promotion of anti-Semitism in any form."
Mear One - whose real name is Kalen Ockerman - has denied being anti-Semitic, saying the mural was about "class and privilege".
Speaking on the BBC's Andrew Marr Show, Mr Watson was shown the mural and asked for his reaction to it.
"My reaction is that is a horrible anti-Semitic mural that was rightly taken down," he said.
But he defended his leader - who said he regretted he "did not look more closely at the image" before commenting online - adding: "You are showing it to me on a 32-inch screen on national television and I have seen it about 100 times on social media.
"It's very different from seeing it on Facebook when you are on the move."
Mr Watson said Mr Corbyn had made his comment in regards to freedom of expression, but apologised for any offence caused.
"I am very, very sorry that people feel hurt by this and that is why I think it is right that Jeremy has expressed regret for it," said Mr Watson.
The deputy leader added that it was time Labour said "enough is enough" and they would "work harder to stamp out anti-Semitism" in the party.
Shadow Brexit Secretary Keir Starmer said the mural was "grotesque and disgusting" but Mr Corbyn had given his explanation for his online comment.
He told ITV's Peston on Sunday: "The most important thing here is that the Labour Party keeps on saying that anti-semitism has no place in our party, in our communities, or in our society.
"We've got to have zero tolerance, and zero tolerance has got to be more than two words. It's got to dictate everything the Labour Party does in relation to anti-Semitism."
Shadow transport secretary Andy McDonald told Sky News that Mr Corbyn "hasn't got an anti-Semitic bone in his body" and that the row had "misinterpreted the intentions of a really good and decent man".
But Ms Berger called Mr Corbyn's response "wholly inadequate".
She tweeted: "It fails to understand on any level the hurt and anguish felt about Anti-Semitism.
"I will be raising this further."
On Friday, Labour MP Angela Smith joined other members in supporting Ms Berger and sent a statement to the Leader's Office, calling for Mr Corbyn to appear before MPs to explain himself.
It read: "It is horrifying that anyone in our party - never mind the leader - should be able to condone anti-Semitism without facing consequences. And rather than facing up, Jeremy Corbyn has chosen to dissemble to defend himself.
"It is simply not credible to suggest that a man with his knowledge of foreign affairs did not recognise those images for what they were.
"Many of us would call for a formal disciplinary process, but the sad truth is that our party has been so badly undermined that no one would believe it would be meaningful."
Strongly agree with @LucianaBerger - am really troubled by the mural, the comments & the way this was handled today. @lisanandy is right - fighting antisemitism is strong part of our tradition & values, and Labour must be better than this https://t.co/INmnUk0K0P— Yvette Cooper (@YvetteCooperMP) March 23, 2018