Ofcom, the broadcast regulator, has declared that the Green party does not have sufficient support to qualify for “major party status” in the general election, but Ukip may have.

Its initial decision makes the Green party’s case to be included in the TV leader election debates much harder to press successfully, although the Greens have until early February to make their case prior to a final decision in early March. It is also harder to mount a successful legal challenge to the broadcasters if they persist in excluding the Greens.

Ofcom stressed that it is for the broadcasters to decide who can participate in the TV leader debates, but this ruling will give broadcasters cover to stick to their position that the Greens should be excluded.

The parties have failed to reach agreement on principles on the conduct of the debates and likely contestants. The delay makes it more difficult to complete arrangements for the debates in time for the election, but not impossible.

Labour and the Liberal Democrats are concerned that the Conservatives will seek an excuse to avoid or reduce the number of debates, and the exclusion of the Greens may give the Conservatives a new argument.

Ofcom in its consultation says the decision will affect not just party election broadcasts, but also the total amount of editorial coverage the parties will receive in the election.

Natalie Bennett, the Green party leader, said the decision was deeply disappointing, stuck in the past, and failed to grasp the way politics was changing fast. It ignored the views of young voters, she added.

In a statement issued on Wednesday morning Ofcom said its initial view was that the Green party (including the Scottish Green party) had not “secured sufficient support in previous elections and current opinion polls to be added to Ofcom’s major party list for the purposes of the May 2015 elections”.

It went on: “The Green party has not demonstrated significant past electoral support in general elections. The Green party has performed better in some other forms of election, such as the 2014 European parliamentary elections, obtaining 8.0% and 8.1% of the vote in England and Scotland.

“In terms of evidence of current support, the party’s opinion poll rating in Great Britain-wide polls has increased in recent months to 5.9% in December 2014 (4.0% on average during 2014).”

In its discussion of Nigel Farage’s party, Ofcom stated: “Ukip has not demonstrated significant past electoral support in previous general elections, achieving 3.5% of the vote in 2010 and has not won a parliamentary seat at a general election.

“Ukip’s performance in a number of other significant forms of election has, however, been stronger. Notably, Ukip has won two seats in parliament at recent byelections.

“Ukip demonstrated a significant level of support in England and Wales in the European parliament elections in 2014 (29.2% England, 27.6% Wales) while its share of the vote was lower in Scotland (10.5%). Ukip also demonstrated significant levels of support in the English local elections (15.7% in 2014 and 19.9% in 2013).

“Opinion poll data indicates that Ukip currently has significant levels of support in England and Wales and has the third highest rating in those polls after the Conservative and Labour parties. Opinion poll data in Scotland shows lower levels of current support.”

The list of major parties 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 said its current list of major Great Britain-wide parties consists of the Conservative party, Labour and the Liberal Democrats.

In Scotland and Wales respectively, the major parties are joined by the Scottish National party and Plaid Cymru.

In Northern Ireland, they are joined by the Alliance party; the Democratic Unionist party; Sinn Féin; the Social Democratic and Labour party; and the Ulster Unionist party.