SIPTU has issued a damning letter to firefighters criticising the "fallacy" of a breakaway union, promising to expel its ringleader and asking members to inform on sympathetic officials.

However, the new union – likely to be called the Irish Association of Front Line Professionals (IAFLP) – is becoming increasingly popular with an estimated potential membership of 2,000.

As first revealed by the Sunday Tribune, firefighters and paramedics around the country are set to join forces with the Psychiatric Nurses Union (PNA) over irritation at the terms of the Croke Park deal.

If the move comes off, the presence of the PNA would render the application for a new union licence – a potentially insurmountable problem – unnecessary.

In its letter, Siptu encouraged Dublin Fire Brigade members to ignore the alternative and promised that efforts are in train to remove its central figure, Paul Shannon.

Encouraging its membership to unite in the face of forthcoming challenges, the letter expresses an awareness that "members of the section are offering members the fallacy of a new union rather than honestly doing the job they were elected to do.

"They continue to try and blame Siptu for all our woes, slamming the Croke Park agreement as the major disaster visited upon by the DFB."

It states that the section committee was informed by its members "of the actions of one of the members of the section who was still publicly encouraging members to sign to a new union."

It stated that although he had been previously sanctioned by the union, he now faces possible expulsion from Siptu "as a consequence of his continued activities".