The Scottish National Party has demanded a rethink of the communications watchdog's "undemocratic and illogical" decision not to list it as a major UK party in a recently-launched consultation.
The SNP's Westminster leader, Angus Robertson MP, has written to Ofcom to formally request the body reconsiders the position, arguing current membership levels of 93,000 make the SNP the third largest political party in the UK.
Ofcom last week published a consultation on the political parties it lists as "major parties" ahead of May's General Election.
The list is important for Ofcom's regulation of election coverage, in particular requiring the relevant broadcasters to allocate at least two Party Election Broadcasts to each major party ahead of an election.
Ofcom keeps the list under review "to ensure it reflects political developments in the UK" and launched the consultation on Thursday to gather views.
While the SNP, the party in power at Holyrood, is listed as a major party in Scotland, it does not feature on the current British list.
The Conservatives, Labour and the Liberal Democrats are on the existing list of major parties in the UK and Ofcom is consulting on whether parties such as the Greens and Ukip should be included. Its initial view is that Ukip may qualify for major party status in England and Wales for the General Election.
But the SNP argues that if it were to continue to be excluded from Ofcom's UK list, it would "cause a democratic deficit in broadcasting for the General Election".
Writing to the Ofcom chief executive, Mr Robertson said: "The recognition of the SNP as a party of majority government and the UK's third largest political party is key in ensuring fair coverage for the general election - yet Ofcom have excluded the SNP from their list of 'major UK parties' even as a basis for consultation, while consulting about parties with far fewer MPs than the SNP has.
"That is quite simply undemocratic and illogical, and it should be unacceptable to any democrat, as it simply doesn't reflect the reality of politics across the UK today."
Mr Robertson also accused Ofcom of failing to keep abreast of political developments since the independence referendum.
"Current Westminster voting intentions put the SNP in the lead in Scotland, and it is clearly wrong that the third biggest political party in the UK should be denied fair representation," he said.
"Broadcasters have a duty to be impartial in their election coverage, and it would be a gross failure in those democratic duties if the SNP were excluded."
Ofcom said that it has set out an initial view based on the available evidence, and has not reached a conclusion. It is seeking views as part of the consultation before making a final statement, due around March.
It stressed that the consultation relates to party election broadcasts, not the proposed leaders' debates. The decision on which leaders are represented in any possible election debates is an editorial matter for broadcasters in agreement with the political parties taking part, not Ofcom.
Broadcasters themselves also have a duty to ensure all coverage is fair.
An Ofcom spokeswoman said: "Ofcom's list of major parties recognises the SNP as a major party in Scotland. This is based on its past election performances, current opinion polling in Scotland and the fact the SNP does not field candidates across other countries of the UK."