Jeremy Corbyn has defended comments he made about a group of British Zionists having "no sense of English irony", saying he is careful with language now.
A 2013 clip was published by the Daily Mail as the Labour leader works to tackle anti-Semitism in his party.
Luciana Berger, a Jewish Labour MP, had said hearing Mr Corbyn's speech made her feel "unwelcome" in the party.
Mr Corbyn said he used the term Zionist in an "accurate political sense and not as a euphemism for Jewish people".
Zionism refers to the movement to create a Jewish state in the Middle East, roughly corresponding to the historical land of Israel, and thus support for the modern state of Israel.
In the speech - made at an event at the Palestinian Return Centre - Mr Corbyn recalled a disagreement between some Zionists and the Palestinian representative to the UK, Manuel Hassassian, following a speech Mr Hassassian had made in Parliament.
He said: "This [Hassassian's speech] was dutifully recorded by the thankfully silent Zionists who were in the audience on that occasion, and then came up and berated him afterwards for what he'd said."
Mr Corbyn, who was a backbench MP at the time, went on to claim that the people concerned "clearly have two problems".
"One is they don't want to study history, and secondly, having lived in this country for a very long time, probably all their lives, they don't understand English irony either."
'Out of context'
But in a statement on Friday evening, Mr Corbyn said he spoke to "defend the Palestinian ambassador in the face of what I thought were deliberate misrepresentations" from people "for whom English was a first language, when it isn't for the ambassador".
He said: "I described those pro-Israel activists as Zionists, in the accurate political sense and not as a euphemism for Jewish people - and that is made clear in the rest of my speech that day.
"I am now more careful with how I might use the term 'Zionist' because a once self-identifying political term has been increasingly hijacked by anti-Semites as code for Jews."
Richard Millett, who believes he is one of the people Mr Corbyn was referring to, said the Labour leader's comments were racist and "deeply antisemitic", calling for him to apologise.
The blogger, who was at the event Mr Corbyn referenced in the speech, said suggesting he did not understand English irony "implies that I'm not from here, not from the United Kingdom. To highlight that, I find very offensive. It was unnecessary to do it, and racist."
A number of other Labour MPs had expressed solidarity with Ms Berger, after she tweeted: "The video released today of the leader of @UKLabour making inexcusable comments - defended by a party spokesman - makes me as a proud British Jew feel unwelcome in my own party.
"I've lived in Britain all my life and I don't need any lessons in history/irony."
Allies of Mr Corbyn said the speech was "taken out of context".
Shadow chancellor John McDonnell said the focus should not be on what Mr Corbyn said five years ago, before he became party leader, but on tackling anti-Semitism in British society, including within Labour.
Mr Corbyn, he said, had devoted his political life to pursuing peace and reconciliation in the Middle East while Labour was committed to playing a "full role" in tackling anti-Semitism within its own ranks and in society.
Meanwhile, a Conservative MP has asked the Parliamentary standards watchdog to investigate Mr Corbyn's remarks.
Helen Grant, the Tory vice-chairwoman for communities, argues they breach the code of conduct for MPs and bring Parliament into disrepute.