The Green party has written to ITN demanding the TV companies rethink their decision to keep the Greens out of the planned election TV debates but include the UK Independence party (Ukip).

The Greens – who are currently polling at around 6-7%, often taking them above the Liberal Democrats – have met broadcasters to discuss their exclusion. They would like to avoid the courts, but feel under an obligation to step up legal pressure.

In a legal letter released on Sunday, the Greens’ solicitors, Leigh Day, warn: “Despite the exchanges between the broadcasters and the Green party it is still entirely unclear what criteria you have relied on to make your decision, and your precise reasons.”

They demand the broadcaster sets out its reasons in writing and also seek a date by which a final decision must be made.

The letter – to Chris Wissun, director of programme compliance for ITV – adds: “The purpose of your debate is not simply to hear the prime minister and the leader of the opposition debate, but to hear the views of a broader range of party leaders, representing the range of parties standing large numbers of candidates across the country at the forthcoming general election. Indeed, this is hardly a controversial position. The prime minister has supported inclusion of the Green party in your proposed debate.”

Leigh Day claims polling for more than a year “has shown the old two/three party system has broken down”. The letter argues: “There is a move from the traditional parties to new and more radical alternatives – which include both Ukip and the Green party. Your role is to make sure that all of the range of views which substantial sections of the public are considering voting for are understood.”

It adds: “Our clients are preparing to commission some polling with an independent polling organisation to find out whether your viewers would wish to have a five-leader debate, including the Green party. We would invite you to agree the questions with us to ensure that you were content that they will produce a fair and reliable result. Our clients will be in contact with you to agree the questions shortly.”

It stresses: “Our client would very much like to resolve this matter without recourse to litigation. However, if your current stance remains in effect, the Green party may be forced to pursue a claim. We assume that you intend to keep this issue under review as negotiations with the other parties continue.”

Green party membership now stands at 27,086 – up 96% since 1 January 2014. At current sign-up rates, approximately 1,000 members are signing up per week and a new member is joining every 10 minutes.