A legal challenge by the Green Party to a decision by RTÉ to exclude it from next week's party leaders' debate is under way at the High Court.

The party argues that its leader, Eamon Ryan, is entitled to be included in the televised debate despite not having the required three TDs elected to the Dáil.

The Greens say the RTÉ rule is unlawful, unfair and overly rigid.

It also argues that the rule breaches democratic values and RTÉ's public service obligations.

Opening the case senior counsel Siobhan Phelan said the decision would result in unfair coverage of pre-election policy, an inequality of treatment of the Green Party which would be to its detriment and would "suggest to the public that the Green Party is not a significant player by its exclusion".

Ms Phelan said the first of the two debates planned by RTÉ was scheduled for Monday and would involve seven leaders, followed by a second debate later in the month between four leaders who will have already participated in the first debate.

She said one of the criteria for inclusion in the leaders’ debate presented a difficulty for the Green Party in that it required the party to have three TDs in the outgoing Dáil, a criterion it was unable to meet, notwithstanding the fact that it was running candidates in all 40 constituencies in the General Election and "is a significant player in political debate in the State."

She said the criteria being applied by the broadcaster had been devised specifically for the leaders’ debate and differed from other elements of election coverage. It had also evolved over time.

She said it appeared the purpose of holding the second debate was to allow a wider platform "which then begs the question why is it being excluded when some parties are being given two opportunities to debate".

This she said "interferes with the democratic process protected under the Constitution".

She said they would be relying squarely on the Broadcasting Act but in a "Constitutional prism".

She said they would point to several different provisions of the Constitution which were engaged including Articles 5 and 6 which refer to the democratic nature of the State and the fact that the State derives its power from the people who elect representatives.

Article 40, which includes the right to free speech and freedom of association, would also be engaged.

It is also claimed the decision amounts to a breach of the broadcasting act which requires the broadcaster to be impartial, objective and fair. The act imposed express duties on RTÉ in this regard, she said.

The proceedings have been brought by Green Party trustee Tom Kivlehan, a former councillor with the party.

In letters to RTÉ, the Green Party complained that recent polls showed the Green Party has three times the support of some of those invited to take part in the debate.

It also complained that some parties did not even exist before the last election and were being treated as if they had an historical mandate.

They pointed out that "no one voted" for some of the parties now included while the Green Party secured thousands of votes in the last election.

It said the "arbitrary criterion" developed by RTÉ permits it to include three new parties in the debate: Renua, the Anti Austerity Alliance-People Before Profit and the Social Democrats while "excluding a former partner in government".

It claimed the number of TDs from the new parties in the outgoing Dáil was more indicative of the strength of the party they belonged to at the time of the last general election than the strength of the party to which they now belong and the new and different policy positions.

The Greens said while it could not satisfy the criterion of three TDs it was a far more established presence on the Irish political scene than several of the other parties who have been invited to participate and the number of TDs in the outgoing Dáil is not a reflection of party significance or support.

RTÉ says criteria is fair, impartial and objective

RTÉ denies all the claims.

In its defence RTÉ says the leaders' debate is just one of many programmes covering the General Election.

It said the method chosen for deciding participation in the debate based on the past electoral performance of a political party was fair, impartial and objective.

It was put in place following an extensive review by a steering committee and applied equally to all political parties in the State.

In letters to the Green Party, the broadcaster said it had never invited anyone to participate in a leaders' debate whose party did not have any TDs in the Dáil.

It also said it could not be obliged to include every party in every programme regardless of the level of Dáil representation.

It did not accept that the leaders' debate had such an effect on voters as claimed by the Green Party and pointed to the fact that United Left Alliance was not included in the last leaders' debate and went on to secure five Dáil seats whereas the Green Party had taken part in the debate and had secured none.

The broadcaster also said the Green Party's performance in the 2014 local elections had been taken into consideration when determining criteria for coverage in all other programmes.

RTÉ argues that election results were a demonstrably impartial way of measuring a political party's support.

Although not invited to take part in the leaders' debate, the party will have an opportunity to make a party political broadcast and will be included in other coverage of the election including having panel members on various discussion shows.

It also contends that RTÉ is not obliged to give the Greens preferential treatment based on its policies, longevity or international standing as to do so would to make a value judgment on the potential longevity or merit of the more recently established political parties which currently have significant Dáil representation.

RTÉ also said it could not be expected to allocate air time on the basis of an acceptance of the importance of a particular policy platform.

The case continues before Ms Justice Marie Baker at the High Court.