David Cameron has been offered the chance to take part in a live digital pre-election debate with the leaders of all the UK’s five main political parties – thereby meeting his demand for the Green party to be included.

A consortium led by the Guardian, the Daily Telegraph and YouTube has invited the leaders to take part in a Digital Debate around the time of the dissolution of parliament at the end of March.

The invitation raises the prospect of reviving the chances of a live leaders’ debate after the prime minister indicated on Thursday that he would not take part in the planned television debates unless the Greens were included.

The Greens had suffered a blow on Thursday in their bid to be included in the debates after Ofcom, the commercial television broadcast regulator, declared in a draft ruling that they do not qualify for “major party status”. The BBC, which is regulated by the BBC Trust, reached a similar judgment. The Ofcom ruling will make it difficult for the Greens to be included in the television debates.

In a letter to the leaders of the five parties – the Conservatives, Labour, the Liberal Democrats, the UK Independence party (Ukip) and the Greens – the Digital Debate consortium says it is not bound by Ofcom rules and is therefore free to invite all five parties to take part in its debate.

The letter says: “We write, following the publication by Ofcom of its draft determination on the major parties list, to invite you to take part in an online debate with the party leaders of the five main UK-wide political parties. As we have outlined previously, an online debate can be flexible as to formats and the number of leaders involved. As such, the Digital Debate is ideally suited to host a debate between the leaders of the Conservative, Labour, Liberal Democrat, Ukip and Green parties.”

The consortium makes clear that it accepts the parties will continue to take part in separate negotiations over the television debates. But it says the Digital Debate would be made available live and after the event to all television networks to broadcast.

The letter adds: “Your participation in the Digital Debate holds many benefits over and above the leaders’ debates held before the last general election 2010. At this year’s general election, the internet will be the main source of news for the majority of UK voters, especially among those aged under 44.”

The Green party accepted the invitation. Caroline Lucas, the Green Party’s MP for Brighton Pavilion, said: “The Green Party would be absolutely delighted to take up this opportunity to reach a wider audience with an important Green message. In light of Ofcom’s ruling yesterday this is an important opportunity to reach a wide audience with an important Green message that otherwise wouldn’t be heard so widely. This is a really crucial way of enhancing the democratic process in Britain. Ofcom’s ruling was not only unfair and undemocratic, it also meant that people in the UK would not have known that there is a genuine progressive alternative to the bigger parties.”

A Labour spokesperson said: “This is an interesting idea and we expect many more proposals to be made by digital platforms in the weeks to come. But to be absolutely clear: none of these should or can be a substitute for live TV debates in the short campaign which in 2010 were watched by more than 20m people and enthusiastically endorsed by all party leaders including David Cameron. The prime minister cannot duck out of this democratic challenge.”

A Ukip source said the party would consider the invitation. The Tories cast doubt on whether Labour would accept the invitation on the grounds that Ed Miliband would be wary of providing the Greens with a platform.