Scotland's burgeoning pro-independence parties have criticised UK broadcasters' decisions to exclude them from national general election debates - and have found an ally in the UK Independence Party (Ukip).

SNP Westminster leader Angus Robertson said the relegation of the SNP from a national platform to a Scotland-only debate is "utterly unacceptable" given that it now has the third biggest membership in UK politics, and particularly because Ukip leader Nigel Farage has been invited to take part.

Green co-convener Patrick Harvie also argued his party should be included, saying it is now on level pegging with the Liberal Democrats in the polls.

Ukip Scottish chairman Arthur Misty Thackery said they have nothing to fear from including the Greens and the SNP in the national debates, and insisted they should reflect a Scottish dimension in the wake of the independence referendum.

The directors of BBC Scotland, Wales and Northern Ireland are writing to the leaders of the main political parties in each nation inviting them to discuss setting up general election debates.

They will be proposing debates during the campaign which would be screened in each nation and would also be available across the UK.

The invitations form part of a wider proposal, announced today by the BBC, ITV, BSkyB and Channel 4 for three Prime Ministerial debates and are "aimed at ensuring impartiality across the UK during the election", BBC Scotland said.

But Mr Robertson hit out: "These proposals will be utterly unacceptable to any democrat.

"What the London-based broadcasters' are proposing fails in their duty to their viewers in Scotland, and simply doesn't reflect the reality of politics across the UK today.

"The broadcasters have the cheek to say that their proposed format factors in 'changes in the political landscape' to justify including Ukip - entirely ignoring the fact that the SNP are now by far the third largest political party in the UK.

"Current Westminster voting intentions put the SNP in the lead in Scotland, and it is clearly wrong that the leader of the third biggest political party in the UK should be shut out of these network debates.

"Yet they are proposing to include Ukip, despite the fact the SNP won six seats at the last general election to Ukip's none - and with the SNP at 40% in Scotland for the general election, to Ukip's 4%.

"Despite the experience of the referendum campaign, the network broadcasters have once again forgotten their responsibility to Scotland. The BBC in particular as Scotland's public service broadcaster should hang their heads in shame - under their proposals, the Tories and Labour would be included in four debates, the Lib Dems three and the SNP just one. That is simply unfair to Scotland's electorate."

The MP continued: "This looks like yet another cosy Westminster carve-up, and we need to know what discussions the broadcasters have had with the other parties, and why there has been absolutely no discussion with the SNP.

"These proposals are clearly inappropriate for Scotland, and cannot stand."

Mr Harvie said: "These debates fail to reflect the truly unique situation Scotland now finds itself in, and we will be making representations to the broadcasters. People who voted Yes and No voted for change, with record numbers joining the Scottish Greens following the referendum.

"Greens across the UK are level pegging on polling with Lib Dems so should not be excluded from high profile TV debates featuring the coalition's austerity cheerleaders. Greens have also regularly been ahead of Lib Dems in polls for the Scottish Parliament, and our membership is now more than twice theirs.

"TV debates are only one element in any election campaign, and Scottish Greens will be harnessing the energy of community activists and social media to get our message across. No matter what further devolution takes place, Westminster is where decisions will continue to be taken on the economy and international affairs, and Green voices need to be heard."

But Mr Thackery said: "We have been excluded from everything for many years.

"We have had to fight tooth and nail to get ourselves accepted on to absolutely everything we have managed to get on, so I say to Angus: welcome to our world, mate."

He insisted: "We would have no fear of the SNP or Greens being included on these debates.

"I think it is also quite relevant that a Scottish dimension is included in the debates, given that we have just had a referendum."

He added: "I would fully expect that David Coburn (Ukip MEP for Scotland) as an elected representative should be included in the Scottish debate.

"We are also currently discussing with the Smith Commission because we have been excluded as it is focusing purely on party representatives in Holyrood, which I think is extremely unfair given we have a party representative in Scotland."

A spokesman for the Scottish Conservatives said: "These debates are for the general election, and will be broadcast to all corners of the UK.

"With the best will in the world, the SNP is not fielding candidates in the vast majority of the country, and Nicola Sturgeon has no designs on being Prime Minister."

He added: "It is likely there will be Scottish-focused general election debates broadcast here, at which point the SNP will have more than enough opportunity to take part."

A BBC Scotland spokesman said: "In Scotland, the BBC is proposing a debate, in peak time on BBC One, involving the leaders of the SNP, Scottish Labour, Scottish Conservative and Scottish Liberal Democrat parties. We have written to the parties today to begin discussions about our proposals and we will ensure impartiality during the election in Scotland. Full details of our content will be released over the coming months once they are finalised."