Labour will crack down on sexual harassment of interns and volunteers to give women “the best start at working life”, Angela Rayner has pledged.

In a speech to the Chartered Management Institute’s (CMI) women’s conference on Thursday, the Labour deputy leader said her party would legislate to make employers liable if they became aware of interns or volunteers being sexually harassed and failed to take action.

She said: “Starting a first job should be one of the most exciting and empowering moments in a woman’s life but instead, all too often it’s the first time they come up against sexual harassment.

“Sexual harassment remains rife in workplaces across Britain. For any employee, but especially interns and volunteers, experiencing sexual harassment can destroy confidence and ruin early careers.”

Labour has already promised to strengthen legal requirements for employers to tackle sexual harassment at work as part of its “New Deal for Working People”, a series of measures intended to overhaul workplace rights in the UK.

On Thursday, Ms Rayner said the party would also extend that duty to ensure it applied to contracted interns and volunteers.

She said: “My message to working women is clear: with our New Deal for Working People, a Labour government will have your back.”

Ann Francke, chief executive of the CMI, welcomed the announcement, saying measures to “strengthen employee wellbeing” were “always a good idea”.

She said: “Good employers recognise that they need to move swiftly and decisively – with clear policies in place when it comes to tackling these behaviours – both for the sake of their teams and also to protect their reputation.”

Sexual harassment at work accounts for around a quarter of all sexual harassment, and is the second most common form after harassment in a public place, according to the Crime Survey of England and Wales.

Research by the Trade Union Congress suggests two-thirds of young women have been sexually harassed at work.

Business minister Kevin Hollinrake said: “Keir Starmer has surrendered to Angela Rayner and the unions.

“Their new French-style laws will ban flexible working, make it harder for small businesses to hire new staff and make it much easier to strike.

“That’s why businesses are warning this risks costing jobs and damaging the economy.”