Jeremy Corbyn will vow to overturn the “warped and degrading culture” that has led to claims of sexual misconduct in Westminster, as he concedes that Labour may have seriously mishandled past allegations.

In an attempt to seize control of the abuse crisis that has hit both the Tories and Labour, he will vow to be part of a campaign to “stand up and say: no more”.

He will say that perpetrators of harassment have been “hiding in plain sight”, but warn that a culture change is needed in Westminster, the media, schools and workplaces across the country. The Labour leader will add that the flurry of sexual harassment claims heralds “a moment of real change”.

Corbyn will also admit his party has failed in the past, after revelations that a prominent activist, Bex Bailey, was told by a party official that reporting she had been raped at a Labour event could damage her career.

“It is not enough to say: this is wrong, then only tinker with procedures,” the Labour leader will tell the party’s north-west regional conference . “How we respond to this moment will shape the way we live our lives. We need to make a fundamental shift in the balance of power, and transform the way our society works.

“Labour is committed not just to challenging a warped and degrading culture in Westminster, and across society, but to overturning it. This week we appointed a leading barrister to investigate if and how the party got it so painfully wrong in the case of Bex Bailey. We are not afraid to turn the spotlight on ourselves.”

His intervention will be welcomed by senior MPs, who have been keen to see Corbyn make a clear statement that there would be a zero-tolerance approach to harassment claims. Some had been concerned about his reluctance to answer questions about claims against Kelvin Hopkins, an ally of Corbyn who has been suspended after allegations of misconduct were made against him.

The allegations were made by Ava Etemadzadeh. She said that following a Labour event in 2013, the MP hugged her very tightly and rubbed himself against her. Hopkins strongly denies the allegations. It is understood the complaints were handed to the Labour whips’ office in 2015, when Rosie Winterton was chief whip. Winterton subsequently made the leader’s office aware of the claims. However, when Corbyn appointed Hopkins to the shadow cabinet last year, the whips were not asked their opinion.

One senior Labour figure said: “I don’t doubt that Jeremy and the leader’s office now fully realise the seriousness of this, but it would appear it was not taken sufficiently seriously at the time.”

The Observer has also been told by several sources that another Corbyn ally bragged about how his friendship with the Labour leader allowed him to get closer to women. There were also multiple claims that he asked a senior female party figure whether she would “twerk against him”. However, no official complaint has been made to the party.

Jess Phillips, chair of the women’s parliamentary Labour party, said she wanted Corbyn to be “front and centre” in condemning abuse and taking action.

“He needs to be clear that there will be zero tolerance of this kind of behaviour to show there is no fear or favour of either members of parliament or members of the Labour movement,” she said.

While the Tories are suffering resignations over sexual misconduct, Labour has been unable to take any advantage due to a number of allegations against its own MPs. These started with claims that Jared O’Mara, MP for Sheffield Hallam, had verbally abused women.

The serious allegations lodged by Bailey, together with a behind-the-scenes campaign by senior female MPs, has led to the announcement of a more robust system for supporting the victims of harassment within the party. Under the plans, an independent specialist organisation will offer advice and support to anyone affected. The body will also offer people a “first step” for reporting any allegations and navigating the party’s procedures. Staff and members of the party’s national executive committee will receive formal training in dealing with the cases.

MPs are keen that the new system gets up and running quickly. However, some in the party believe it does not go far enough because complaints will not be investigated by an independent body. Jasmin Beckett, a member of Labour’s ruling national executive committee, is among those to express concern.

Meanwhile, Clive Lewis, seen as a potential future leader, has denied a claim that he groped a woman at the Labour conference in September. The party is investigating a formal complaint against him. Former Foreign Office minister Ivan Lewis, MP for Bury South, has denied he had made non-consensual sexual advances toward women.