The leaders of Plaid and SNP question the BBC's impartiality
Plaid Cymru and the Scottish National Party (SNP) claim the BBC's live Prime Ministerial TV debates could "endanger the conduct of a free election".
The claim was made in a joint letter from the parties' leaders to BBC Director General Mark Thompson.
Ieuan Wyn Jones and Alex Salmond are angry their parties will not be represented in three debates to be televised on BBC, ITV and Sky.
The BBC argues it has clearly set out how impartiality will be achieved.
Mr Jones and Mr Salmond have also written to international press bureaux heads accusing the BBC of bias.
It follows plans for Prime Ministerial debates on the BBC, ITV and Sky between Labour Prime Minister Gordon Brown, Conservative leader David Cameron and Liberal Democrat leader Nick Clegg in the run up to the general election.
Plaid and the SNP are angry that they will not be represented in these debates, claiming the proposals exclude three of the four countries which make up the UK and parties of government in Wales and Scotland.
Plaid Cymru and the SNP have expressed anger over their exclusion from the prime ministerial debates
They've written a joint letter to BBC director general Mark Thompson arguing the BBC's plans deny "fair competition of ideas" and "could endanger the conduct of a free election".
They write: "For politics to be about healthy debate, the winning of hearts and minds, rather than about prior tribal political affiliation, then parties in reasonable competition must be given a similar opportunity to present their policy platform to the electorate.
"The medium of television has a unique ability at election time to bring the competition of ideas, which is at the heart of the democratic system, into the living room of every voter in the country.
"Without a properly informed electorate, the conditions for a free election are not possible."
'Debates breach impartiality'
In a separate letter to the international press, the two leaders state that the BBC's proposal breaches the corporation's duty of impartiality and "effectively disenfranchises the people of Scotland and Wales".
They write: "As yet, no satisfactory measures have been put in place to ensure... balanced coverage, reflecting the different political make-up of the United Kingdom."
The leaders argue other broadcasters around the world achieve balance in more complex political situations than the UK's and say they're disappointed "the BBC lacks the ambition to even try".
The debates will feature Nick Clegg, David Cameron and Gordon Brown
Ieuan Wyn Jones said: "Unless broadcasts are held in a fair, impartial and accurate way, the legitimacy and integrity of the general election results will be called into question. That cannot be an acceptable state of affairs for anyone.
"The action the BBC has taken in putting together these plans is nothing short of a betrayal of the principles upon which the editorial integrity of the BBC is built."
Alex Salmond argued that after "almost 90 years of being a world respected independent broadcaster" the BBC had "allowed itself to become politically compromised" in setting up the debates.
A BBC spokeswoman said the corporation had clearly set out the agreement reached on the televised Prime Ministerial Debates including how impartiality will be achieved.
She added: "The BBC has also announced it is to hold subsequent leaders' debates in Scotland, Wales and Northern Ireland, part of a range of measures to ensure that the SNP, Plaid Cymru, Northern Ireland and other parties have appropriate opportunities to be heard.
"We will continue to speak to the parties as we develop our plans."
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