British opposition leader Jeremy Corbyn risked further souring his relationship with the Jewish community by attending a Passover event with a left-wing Jewish group that has called for the destruction of Israel.
Jewdas has also hosted a fringe-left alternative to the popular Birthright Trips to Israel, dubbing its trips to Spain "Birthwrong".
But Mr Lansman told BBC Radio 4's Today programme: "Well, it's certainly not helpful to Jeremy or the cause of opposing anti-Semitism in the Labour Party as it happens ... and I think the important thing is that Jeremy is seeking to meet with mainstream Jewish organisations".
He told reporters: "It wasn't a meeting last night, it was a Seder event, which is a celebration of Passover, which I celebrate with young Jewish people from my own community and my own constituency".
Labour MP John Woodcock, a leader in the Labour Friends of Israel group, said Corbyn's attendance was "irresponsible and unsafe".
'He wrote to the Board of Deputies and Jewish Leadership Council last week to ask for an urgent formal meeting to discuss tackling anti-Semitism in the Labour Party and in society'. In a statement on social media, Labour MP John Woodcock said meeting the group gave the message that "extreme views are OK".
Some critics believe Mr Corbyn's association with Jewdas is a sign that he is not committed to tackling anti-Semitism.
Jeremy was invited to a Passover Seder with Jewish members of his local community, said the spokesman.
After the Seder, Jewdas tweeted that "tonight reminded us that the reasons we're Jewish are the same as our reasons for being revolutionaries: our commitment to making this world better than it is now". "Please feel free to make a donation!"
A Labour candidate in Kent has been dropped from the party after allegedly sharing posts on Facebook deemed to have been anti-Semitic.
'We want everyone to know - we're leaving Mitzrayim (Egypt).
It said: "Labour takes all complaints of anti-Semitism extremely seriously, which are fully investigated and appropriate disciplinary action taken in line with our rules and procedures". Change is coming. Revolution will come'.
Mr Arkush also claimed Jewdas are "not all Jewish" and branded the group "lifelong campaigners against the Jewish community to whom they show the upmost disregard and contempt".
Mr. Corbyn, he added, "is either incredibly obtuse or he just doesn't care", and he has created a crisis he could have averted simply by "taking the advice of people he doesn't like".
Sir David Garrard, who donated nearly £2m to Labour in the 18 months before the 2015 general election, left Labour over the weekend as the party remains embroiled in an anti-Semitism row.
It was "a very clear two fingered salute at mainstream British Jewry", according to Gideon Falter, the chairman of the Campaign Against Antisemitism.
Blair served as British Prime Minister between 1997-2007, and led the Labor party for 13 years.