More than 60 Labour peers have taken the extraordinary step of taking a full-page advert in the Guardian to accuse Jeremy Corbyn of “allowing antisemitism to grow in our party and presiding over the most shaming period in Labour’s history”.

Representing roughly a third of Labour’s members in the House of Lords, the signatories added that Mr Corbyn had “failed the test of leadership” over antisemitism.

The statement was supported by a number of former government ministers, including Beverley Hughes, John Reid, Jack Cunningham, Peter Mandelson and Peter Hain.

It comes amid a backlash within the party after last week’s Panorama exposé, which highlighted interventions by Mr Corbyn’s aides in the handling of antisemitism complaints.

The 64 peers argued that the tactic of Mr Corbyn’s office and his followers was to “deny the truth of the message and shoot the messengers”, making reference to complaints raised by party members and whistleblowers.

The advert reads: “The Labour Party welcomes everyone* irrespective of race, creed, age, gender identity, or sexual orientation. (*except, it seems, Jews)

“This is your legacy Mr Corbyn… We are not asking if you are an antisemite. We are saying you are accountable as leader for allowing antisemitism to grow in our party and presiding over the most shaming period in Labour’s history.

“You still haven’t accepted your responsibility. Labour can’t fight racism in society if we can’t deal with racism in our own party. We can’t be a credible alternative government that will bring the country together if we can’t get our own house in order.

“But we cannot stay silent while younger, braver party workers and members speak out. You have failed to defend our party’s anti-racist values. You have therefore failed the test of leadership.”

Responding to the advert, a party spokesman told the Guardian that Mr Corbyn and Labour “stand in solidarity with Jewish people and are fully committed to the support, defence and celebration of the Jewish community”.

The spokesman said Labour was taking decisive action against antisemitism “regardless of false and misleading claims about the party by those hostile to Jeremy Corbyn’s politics.”

He added: “Jeremy Corbyn has made clear in interviews, videos and articles that there is no place for antisemitism in the party.

“[General Secretary] Jennie Formby has sped up and strengthened procedures and the rate at which cases are dealt with has increased more than four-fold. Since September 2015, the number of cases that have undergone disciplinary procedures relate to about 0.06% of members.

“The Labour party’s comprehensive rebuttal of the Panorama programme did not in any way criticise Jewish members who have suffered antisemitism. As we said after the programme aired, we will fully investigate any complaints alleging antisemitic incidents reported by party members in interviews in the programme.”