About 1,300 offshore oil and gas workers have begun a major 48-hour strike.
Unite has described the action as the “biggest stoppage in a generation”.
The union warned of a “tsunami” of industrial unrest in the row over jobs, pay and conditions.
Five operators are involved in the ongoing dispute, but strike action at Worley Services has been suspended to allow for further talks with the company.
The other companies directly involved are Bilfinger UK, Petrofac Facilities Management, Stork Technical Services and Sparrows Offshore Services.
Unite said the strike action would hit operators including BP, CNRI, EnQuest, Harbour, Ithaca, Shell, TAQA and Total.
The workers include electrical, production and mechanical technicians in addition to deck crew, scaffolders, crane operators, pipefitters, platers, and riggers.
Unite industrial officer John Boland said: “The 48-hour strike action will cause severe problems for contractors and operators.
“We believe that due to the specialised nature of work our members undertake, offshore platforms will not be able to operate safely and efficiently without them.
“The strike action could halt production on dozens of platforms and bring them to a standstill.”
‘Constructive dialogue’
About 700 offshore workers at Bilfinger are involved, along with more than 360 at Stork, 150 at Sparrows Offshore Services and about 100 more at Petrofac and Worley Services.
Stork said: “We respect the right for peaceful and planned strike action.
“As an employer and a signatory of the Energy Services Agreement, we remain focused on engaging with all stakeholders to reach a resolution that protects and sustains employment opportunities, supports safety and operational continuity. This remains our absolute priority.”
Bilfinger UK said: “Operational safety remains our top priority and we have procedures in place to minimise any potential disruption.”
Shell said: “We support constructive dialogue between the unions and contracting companies to reach a mutually agreed resolution.”