Lib Dems: We weren’t invited to BBC debate – BBC: All parties agreed to the format

But according to the BBC, the format of last night's so-called challengers' debate – featuring Labour, the Greens, UKIP, Plaid Cymru and the SNP – was agreed in a deal between broadcasters and all the political parties.

The party’s leader, Nick Clegg, said on Twitter he “would have happily taken part and proudly defended our strong record in government” had he received an invitation.

The official Liberal Democrats Twitter account also emphasised repeatedly that it had not been the party’s choice to miss the debate. It said: “Nick Clegg has made it clear he would go to any TV debate he was invited to.”

This was in response to a number of Twitter users questioning Clegg’s absence.

Wow,I was half tempted to vote for Clegg but if @LibDems cant be bothered to show up can't see me being bothered to vote for them #BBCDebate

— Mandiii (@mandisturridge) April 16, 2015

David Cameron also did not attend the debate, having agreed to take part in only one election debate with seven party leaders.

A BBC spokesperson said that the “agreed” debate formats had “involved all sides […] making a number of compromises”. They said: “Over many months the broadcasters, jointly, have worked hard to ensure that their audiences were offered the best possible combination of programmes to help them engage with the election, to inform them about the issues and to scrutinise the politicians.

“The programmes which have been agreed – including the BBC debate on the 16th – involved all sides, broadcasters and parties, making a number of compromises, allowing the different elements to go ahead.

“For each of the broadcasters, ensuring due impartiality is not only a priority, but an obligation and the BBC is satisfied that the election programmes it is offering on each of its services fulfils that obligation and that all the relevant parties will have the opportunity to put their case – and to be appropriately scrutinised.”

Instead of the debate, Clegg was hard at work campaigning, “canvassing” at the Devonshire Arms in Ecclesall.

Earlier this year David Cameron refused to take part in any debate which involved UKIP and not the Greens. This led broadcasters to propose two seven-way debates, and one head-to-head between Cameron and Ed Miliband.

Last month Cameron made a final offer saying that he would take part in one debate, involving six other party leaders.

The final compromise agreed with broadcasters saw this take place as well as last night's 'challengers' debate (excluding the two government parties). There will also be a special edition of BBC One's Question Time featuring Miliband, Clegg and Cameron. In addition to this, Cameron and Miliband were interviewed by Jeremy Paxman for Channel 4/Sky News.