An indefinite strike by hundreds of workers at the Cadbury production plant in Coolock, Dublin began this morning in a dispute over the outsourcing of jobs.
Cadbury plans to outsource 17 positions in its stores division as part of a Labour Court recommendation, which was rejected by Unite and SIPTU.
Both unions now say they will strike until the company rescinds the decision to outsource.
The owner of Cadbury, Mondelez International, has said the strike action will undermine its future viability as well as its operation in Rathmore, Co Kerry.
However, all sides in the dispute have accepted an invitation to attend talks at the Workplace Relations Commission tomorrow morning.
The unions represent around 350 of the 900 employees at the Coolock plant.
WRC director general Kieran Mulvey said he is fearful for the future of Cadbury in Ireland and that the company's threat of disengagement was not an idle one.
Mr Mulvey said the situation was extremely serious and in his view, far more serious than the Luas dispute.
He said the dispute centred on 700 jobs at two plants but also all the feeder businesses to those plants.
Earlier, SIPTU Manufacturing Division Organiser Gerry McCormack said: "The attempt to outsource these jobs is seen as a further erosion of the viability of the facility.
"It follows the movement of the production of the Time Out bar from this plant to Poland."
Mr McCormack added: "Workers at the plant have over recent years agreed to the major restructuring of operations and changes in work practices."
"However, the attempt by the management of Mondelez International to further downgrade the plant's operations has caused outrage and increased fears among the workforce about its long-term commitment to the facility."
He said that had left the workforce with no alternative but to strike.
Mondelez facing 'intense pressures'
In a statement, Mondelez Ireland said: "In line with many other manufacturing businesses in Ireland, both the Coolock and Rathmore chocolate manufacturing sites are under intense pressures from international competition.
"In February 2015, Mondelez Ireland presented proposals to our employees in Ireland to restructure our chocolate manufacturing business, with the aim of streamlining it to become more sustainable and competitive for the future.
"These proposals included an €11.7m investment in new chocolate-making technology to enable our Coolock plant to concentrate on core chocolate brands Cadbury Dairy Milk 8-square, Flake, Twirl and Boost, for consumption in Ireland and the export market."
The statement adds: "This action is being taken in protest at the planned future outsourcing of a small number of non-core roles at some point in the future.
"The company deeply regrets that SIPTU and Unite have decided to take this step.
"Any form of industrial action only further undermines the future viability of both the Coolock and Rathmore manufacturing sites and this action ultimately endangers the jobs of the circa 700 people who are employed within the business.
"The management team remains available to meet again with the SIPTU and Unite representatives to attempt to find ways to resolve this dispute and to work together on building a more sustainable future for the Coolock and Rathmore sites," the statement said.