Rupert Murdoch's empire has to be challenged, says Harriet Harman

Labour calls on parties to act together on eve of MacTaggart Lecture by Elisabeth Murdoch

There were renewed calls for greater limits on the Murdoch family's power in the British media last night as Rupert Murdoch's daughter Elisabeth prepared for a keynote speech sure to be scrutinised for even the subtlest clues about News Corp's future.

The Labour deputy leader, Harriet Harman, said she would make an "open offer" to the Conservatives and the Liberal Democrats for talks on tightening restrictions on media ownership in the UK, saying it was time "to deal with the issue of the invincibility of the Murdoch media empire".

Ms Harman's comments, made in an interview yesterday, were pointed in their timing – coming on the eve of the annual MacTaggart Lecture to the Edinburgh International Television Festival, delivered this year by Ms Murdoch at a delicate time for her family's position within News Corp.

Three years after her younger brother James used the same platform to make a scathing attack on the ambitions of the BBC, Ms Murdoch's moment in the spotlight will be analysed by shareholders ahead of their key meeting in Los Angeles in October, when her father will come under further pressure to reduce his role at the head of the company.

Ms Murdoch's position has garnered greater significance after James – who was seen as the heir to his father's throne – resigned his position as chairman of BSkyB in April, damaged by his handling of the phone-hacking scandal at News International.

Even if Ms Murdoch chooses to minimise her references to News Corp, there are those who will detect a determination to distance herself from the hacking scandal in order to improve her longer-term prospects within the family business.

However, Ms Murdoch also carries on her shoulders a weight of responsibility to large sections of her colleagues in the industry. She is the first woman to deliver the prestigious address for 17 years, since the broadcaster and Independent on Sunday columnist Janet Street-Porter in 1995. With that in mind, some will therefore want her to use the MacTaggart to highlight the lack of women at the very top of the British television industry, especially after the BBC recently dashed hopes it would be appointing its first female director-general.

She is also the first representative of the independent sector to give the MacTaggart since the former Endemol UK chief Peter Bazalgette in 1998. Other members of her audience will therefore be hoping she speaks up for the British independent television sector, which she has helped to develop since founding Shine in 2001.

The company, which has made MasterChef into a global success and produced other hits including Merlin, is one of a series of British-based "super-indie" companies that have transformed the television industry. Indeed, Shine's success was so great that it was bought last year – by News Corp.