Up to 4,000 plumbers, fitters and welders belonging to the Unite trade union have voted to take strike action if employers try to reduce their pay and conditions, or fail to implement a 2.7% pay increase due from today.
This follows a High Court ruling earlier this summer that legally binding Sectoral Employment Orders negotiated between employers and unions on pay and conditions were unconstitutional.
Other unions in the construction sector are holding similar "precautionary" ballots, the results of which are due on 14 September.
The constitutional challenge to SEOs was taken by electrical contractors, and after they won their High Court case, a proposed SEO for the electrical contracting sector was immediately deemed void.
However, Mr Justice Garrett Simons deferred an order of unconstitutionality for two other SEOs covering the mechanical sector (including plumbers, fitters and welders) and the construction sector pending an appeal by the Government.
Following the ruling, Unite, Connect, SIPTU, BATU and OPATSI organised "protective" ballots for industrial action to take effect if employers sought to reduce pay and conditions on the basis that the collective SEO was invalid.
Unite is the first union to announce its ballot result for mechanical workers, which was "almost unanimous".
The union points out that under the mechanical sector SEO, a 2.7% pay rise is due to take effect today.
Regional Officer Tom Fitzgerald said Unite was monitoring the situation to ensure that all employers adhere to the agreement.
"Unite has consistently argued that sectoral collective agreements provide a level playing field for employers and a vehicle for workers to negotiate decent pay and conditions," Mr Fitzgerald said.
"We have written to employers' bodies rejecting their calls in mid-August for a deferral of the increases, and we have informed them that any employer failing to implement the pay increases due to come into force today will be deemed to be in breach of negotiated agreements," he added
Mr Fitzgerald said employers had a simple choice, they could either abide by all agreed terms and conditions, including scheduled pay increases, or they could face determined industrial action.
The result of a further Unite ballot of non-mechanical construction workers will be revealed on 14 September.
Meanwhile, the Connect trade union, which represents workers in the construction (4,500), electrical contracting (13,000) and mechanical contracting (6,000) sectors, will have its ballot result in two weeks' time.
General Secretary Paddy Kavanagh warned there would be "all out war" if members did not receive their pay rise, or if any attempt were made to cut their pay and conditions in the absence of an SEO.
SIPTU is also balloting for strike action if the current SEO for the general construction sector is breached.
SIPTU Organiser John Regan said his union expected the 2.7% increase due under that SEO on 1 October to be paid.
The Construction Industry Federation said that increasing labour costs in the current environment would be "totally unsustainable".
Director General Tom Parlon said that because of Covid-19, companies were working in very difficult circumstances to keep sites open, ensure they were safe, and keep the workforce in employment.
He said that increasing the cost of construction would discourage future investment and result in the loss of jobs.
Mr Parlon said that when the economy regains "a level of stability and certainty", the issue of pay rises could be reviewed.
He called for immediate dialogue with unions "as a matter of urgency".