A proposal for a party leaders' 2015 election debate to be staged on the internet is being launched by a unique alliance of YouTube, the Guardian, and the Telegraph.

The initiative for a debate broadcast through YouTube would break the monopoly of existing broadcasters and allow innovative forms of audience participation for the political parties. The proposal, using the hashtag #onlinedebate, has been in preparation for months and was sent to the main party leaders on Thursday following informal talks with politicians.

The cross-platform collaboration would put an online debate before the vast majority of the UK's 55 million internet users, and would be aimed at engaging a younger and more interactive audience.

A live feed could be embedded on any media site and be carried by any broadcasters on traditional TV and radio.

Until now, there has been an assumption that any 2015 election debates will be screened by the existing range of TV channels. But the new alliance claims this is to misunderstand the degree to which traditional boundaries between TV and the web, including newspaper websites, have eroded. For those aged under 44 the internet is now the main source of news, the three news media groups say, arguing that the format would reach an audience often alienated by traditional broadcasters.

It would give those voters more opportunities to engage with debates through tweets, likes, shares or questions to politicians. The format would also make it easier for the political parties to set up arrangements for viewers to make contact or donate.

The three-pronged consortium argues that the experience in America shows elections, including leader debates, are increasingly being fought out online, rather than on the old TV networks. Videos uploaded to YouTube tagged Obama or Romney were viewed 2.7bn times during the 2012 election cycle.

David Cameron, Nick Clegg and Gordon Brown at one of the three leaders' debates before the 2010 general election. Photograph: Getty Images

The consortium of YouTube, which is owned by Google, the Guardian and the Telegraph, combining the left and right in UK politics, has given a commitment that the moderator in any debate would be a woman. All three moderators in 2010 on the traditional TV channels were men – Adam Boulton, Alastair Stewart and David Dimbleby.

YouTube, the Guardian and the Telegraph say they are open-minded about the precise format for the debate, including the complex issue of which leaders would take part and in what combination.

One advantage for the websites is that they are not as curtailed by strict broadcasting rules about the format and so could offer a more flexible alternative.

Alan Rusbridger, the Guardian editor-in-chief, said: "The digital world has become an increasingly vital democratic tool and forum for debate, and it's imperative that politicians understand and embrace the opportunities afforded to them by it. Open, digital-first journalism that holds power to account lies at the heart of the Guardian's mission, which is why we believe the proposed partnership between us, the Telegraph and YouTube is such an important and exciting next step in the changing relationship between politicians and the electorate."

Jason Seiken, editor-in-chief of Telegraph Media Group, said the advantage of the internet was that the TV debates could become a "two-way conversation" between viewer and politician.

He said: "The power of digital media comes in its ability to connect our leaders and the public in a two-way conversation. It appeals to millions who have not previously bothered to engage in the political process."

Peter Barron, a former editor of BBC2's Newsnight and now head of communications for Europe at Google, added: "Given the way voters are consuming and engaging with news these days we think it's time for an online election debate."

In a first for British politics the three party leaders in 2010 held three TV debates, but since then David Cameron has voiced doubts about the format saying they drained the oxygen out of the rest of the election campaign.

Broadcasters themselves have admitted to surprise at the extent to which the debates resonated and admit the focus on ensuring the debates happened led to a lack of imagination about the format, and instead a determination to simplify.

The Labour election campaign co-ordinator Douglas Alexander has suggested a repeat of the 2010 format in which the three leaders debate three times, first on Sky, then BBC and finally ITV. Channel 4 News was excluded from the arrangement, but is now likely to mount a joint bid with Sky.

Nick Clegg has warned that if Cameron tries to exclude him from the debates, he will mount a legal challenge.

A House of Lords committee said in a report this week that there was clear public support for more televised debates next year but warned that a "whole range of obstacles" stood in the way of the exercise being repeated.

The committee, headed by the Tory peer Lord Inglewood, called on broadcasters to do more to encourage voter participation in the elections and create an online "hub" where the debates could be watched on demand.