Staff in Bord Fáilte and CERT say they will not co-operate with the planned merger tomorrow of the two bodies into a new national tourism organisation.
SIPTU, which represents most of the staff in both existing organisations, has also threatened industrial action if the merger is pushed through without agreement.
The new body, Fáilte Ireland, is to be responsible for marketing and developing the tourism industry at home. The Government announced last week it would be chaired by the businesswoman, Ms Gillian Bowler.
Three weeks of talks on the pay and conditions for staff, however, ended without agreement last Friday.
SIPTU expressed anger yesterday that the merger was to go ahead in the absence of agreed terms and conditions.
Mr Brendan Hayes, regional secretary with the union, said it had asked the Minister for Arts, Sport and Tourism, Mr O'Donoghue, to allow a "short extension" of time so that talks could be completed.
The Minister's announcement that the merger would take place on May 28th had, however, made the task of reaching agreement more difficult, he said.
In a letter last Friday to the chief executive-designate of the new body, Mr Shaun Quinn, the union said it had made it clear that its members would "cease all co-operation with the establishment of the new authority and have no involvement with any matters associated with it".
About 200 of the 230 staff employed by Bord Fáilte and CERT are members of SIPTU.
A spokesman for the Department of Arts, Sport and Tourism said it did not comment on industrial relations matters, but said no one's pay and conditions would be "any less beneficial" as a result of the merger.
Fáilte Ireland, in a statement, said that while the absence of staff co-operation was a "complicating factor" in its establishment, it did not foresee any disruption to services to industry.
Ms Patricia King of SIPTU said it was balloting members on industrial action in the event of new pay or conditions being imposed on them.