The Labour leadership candidate Emily Thornberry has given Jeremy Corbyn “0 out of 10” for his performance during the general election.

The shadow foreign secretary gave Corbyn 10 out of 10 for principle but said Labour lost “very, very badly” at the polls last month, arguing that this was the most important criteria by which to judge the party’s leader.

Thornberry said she was also convinced that the departing leader was not antisemitic but gave him 2 out of 10 for his ability to stamp out anti-Jewish racism from the party.

Her response contrasts with that of the shadow business secretary and fellow leadership hopeful, Rebecca Long Bailey, who said on Tuesday that Corbyn deserves 10 out of 10 as a leader.

Thornberry, the MP for Islington South and Finsbury, may struggle to obtain the backing of 22 MPs and MEPs required to move on to the next stage of the leadership contest.

Over the last 24 hours, she has taken the party leadership to task for its failings during the election and over antisemitism in a series of outspoken comments.

“Losing an election is probably the most important thing by which you would measure a leader, and obviously we lost very, very badly. And on that basis, I would give him on that nought out of 10. What else could I do?” Thornberry told Sky News.

“But when it comes to principle, bringing people into the party, firing up the party, bringing us back to speaking from our hearts, I would give him 10 out of 10.”

On Wednesday, Thornberry accused Corbyn’s advisers of trying to drop references to Palestinian rocket attacks against Israeli civilians from the party’s 2019 election manifesto.

The shadow foreign secretary said she was left “deeply disturbed” by the move she viewed as evidence that the Labour leader’s key lieutenants were happy to tolerate the deaths of Israeli citizens.

“Disgustingly, attacks on Israeli civilians were being deliberately dismissed in a way that would never have been tolerated of attacks on any civilians in any other country around the world,” Thornberry wrote in a comment piece for Jewish News.

She claimed “Jeremy’s office” dismissed her repeated complaints about the planned deletion. In an email exchange, it was suggested that the decision was “balanced, considering the considerable imbalance in the conflict”.

The alleged comments were made at a time when Corbyn’s advisers were already fighting off hundreds of allegations of antisemitic incidents within the party. Thornberry will now face pressure to name the aides who made the alleged comments.

Thornberry said that, if elected, she would adopt recommendations made by the Board of Deputies of British Jews, the Jewish Labour Movement and the equalities watchdog, which is investigating allegations of antisemitism within the party.

“As a veteran of the bitter struggle inside the shadow cabinet to get us to endorse the IHRA [International Holocaust Remembrance Alliance] definition of antisemitism, and all its examples, I know how hard these fights can be,” she said.

“And we then need to get down on our hands and knees to the Jewish community and ask them for forgiveness and a fresh start.”

It has also emerged that prosecutors are considering whether to bring charges against five people as part of an investigation into alleged antisemitism by members of the Labour party.

Four people were arrested and two interviewed under caution last year as part of the inquiry. Investigators have sent five files of evidence to the Crown Prosecution Service.

Labour leadership candidates need nominations from 10% of Labour MPs and MEPs, who total 212, making the required threshold 22.

Contenders who receive enough nominations will enter phase two of the contest, where they have to secure the backing of 5% of constituency Labour branches or at least three affiliate groups, two of which must be made up of trade unions, to get on the final ballot paper to go to the membership.