Controversial anti-Lisbon treaty group Cóir is refusing to say how many registered members it has, despite regularly claiming it has around 2,600 "volunteers and supporters" around the country.

This means it is alone among six leading Lisbon treaty campaign organisations in failing to provide this information to the Sunday Tribune. But all of the campaign groups declined to make their membership registers available, citing data protection concerns.

Figures provided by the five other organisations , which represent both the Yes and No camps, reveal that they have fewer than 1,600 registered members between them. The figures do not include cases where one individual is a member of several organisations.

Asked for details of its membership, as opposed to supporters or volunteers involved in Cóir's anti-Lisbon treaty campaign, spokesman Richard Greene repeatedly answered that it has 2,600 "volunteers and supporters".

Despite numerous attempts to establish how many of these are registered members of Cóir – with the power to influence and shape its policies as opposed to simply implementing them – he refused to elaborate further.

"Cóir has 2,600 volunteers and supporters. Most of those would also donate to the campaign; 2,600 is an accurate estimate from our database and the people who came to public and private meetings around the country," he said.

Roger Cole of Peace and Neutrality Alliance (Pana) revealed it has just 105 people on its membership database, adding that it does not actively seek members and has a large number of affiliate organisations. He acknowledged that not all of these would "agree with Pana on every issue".

The Ireland for Europe campaign, whose patrons include U2's the Edge and BP chairman Peter Sutherland, has just 50 registered members but estimates that it has at least 500 supporters and volunteers in some 20 groups around the country.

A spokesman for Libertas said it has 547 members, up from around 340 when it registered as a political party earlier this year.

Women for Europe said it has around 400 members and Generation Yes has 491 current members.

Many of the organisations surveyed claim to be "people-based" movements, although only Libertas is a registered political party, having previously been a single-issue campaign group. For example, Cóir describes itself as a "grassroots organisation" which "spoke to the people for the people" in the last Lisbon campaign, while Generation Yes describes itself as a "movement of young people from all over Ireland", and Ireland for Europe labels itself a "people's movement".