The leaders of Plaid and the SNP question the BBC's impartiality
The SNP and Plaid Cymru are preparing for a potential appeal against the BBC's decision to exclude them from the prime ministerial debates.
The Welsh and Scottish nationalists wanted their leaders to be included in the live TV clashes between Gordon Brown, David Cameron and Nick Clegg.
The BBC Trust has now written to them, asking them to prepare for a possible oral hearing between 22 and 27 April.
The trust says it will first consider whether the appeal is admissible.
Three prime ministerial debates are being staged by the BBC, ITV1 and Sky in the run-up to the election on 6 May.
The first one, with a themed section on domestic policy, was held on ITV1 on Thursday. Nick Clegg was widely perceived to have done well, with a subsequent surge for the Lib Dems in the polls attributed to his showing.
The second will be broadcast by Sky on Thursday 22 April: its theme will be international affairs.
The third debate will be on BBC One on Thursday 29 April, with the themed section spotlighting the economy.
But Plaid Cymru leader Ieuan Wyn Jones and the SNP's Alex Salmond are not involved in any of the debates.
'Excluded from discussions'
They say the debates exclude three of the four countries which make up the UK, and parties of government in Wales and Scotland.
They complain that viewers are being misled in the two countries, where domestic issues such as health and education are devolved.
Welsh deputy first minister Mr Jones took part in a Welsh leaders' debate on Sky News on Sunday, when he locked horns with Welsh Secretary Peter Hain, shadow Welsh secretary Cheryl Gillan and Welsh Liberal Democrat leader Kirsty Williams.
Plaid chief executive Gwenllian Lansdown said: "We are ready to meet the BBC Trust at any time to discuss our appeal.
"The exclusion of Plaid and the SNP, parties of government in both Wales and Scotland, from the election leaders' debates is fundamentally undemocratic.
"We believe it is a clear breach of the BBC's obligation of impartiality."
SNP chief executive Peter Murrell added: "We also believe that the BBC executive has failed in its duties by excluding the SNP and Plaid Cymru from the negotiations which were held with the other parties over what was clearly a prolonged period.
"Those discussions effectively allowed the UK parties to dictate the format of the leaders' debates to the broadcasters."
Ad hoc committee
The BBC has argued that it has clearly set out how impartiality will be achieved.
A BBC Trust spokesperson said on Sunday: "We can confirm that we have written to Plaid Cymru and the SNP to inform them that their appeal to the trust will be considered by an ad hoc committee of the trust.
"As with all appeals, this committee will first consider if the appeal is admissible. If it is, the committee will consider whether an oral hearing is necessary.
"If it is not necessary, the committee will then hear the admissible parts of the appeal at the same meeting.
"If the committee considers that an oral hearing is necessary, the committee will reconvene at a later date to allow for this.
"As with all appeals, the trust will publish the outcome of its decision."