Contract workers at the Sellafield nuclear site in Cumbria, Heysham nuclear power station in Lancashire and Staythorpe power station near Newark in Nottinghamshire were among those taking unofficial action for the first time.
One union official called on every trade unionist in the construction industry to join the series of walkouts in protest at the hiring of Italian and Portuguese workers at a power station in Lincolnshire.
Meanwhile, the start of crucial talks aimed at resolving the row was set to be delayed because of the extreme weather, which caused chaos to Britain's transport network.
Business Secretary Lord Mandelson maintained that UK firms and workers were not being discriminated against and called for the unofficial strikes to stop.
Lord Mandelson said he understood the concerns of British workers, but he stressed that Total, which runs the Lincolnshire power station at the centre of the row, had refuted claims that UK workers had been excluded from contracts.
"I hope in the light of that people will be reassured and call off these unofficial disputes."
He added that the contract at the centre of the dispute was originally awarded to a British firm but it did not fulfil it so it was given to an Italian company which then drew on its own workforce.
The minister also said workers at another site owned by Total had voted not to join the unofficial strikes.
He stressed that under EU law companies had the right to sub-contract work to those companies "best suited" for the job.
He told BBC Radio 4's Today programme that claims that British workers had been excluded from the disputed contract, or that foreign workers were being paid less than the going rate, were both unfounded.
Management at Sellafield said the contractors who walked out this morning were building new storage facilities at the site.
Bosses from Total will meet sub-contractors and union leaders for hastily-arranged talks in Scunthorpe.
Moves are also under way to set up a special panel, under an independent chairman, to review the recruitment of the hundreds of Italian and Portuguese workers on the £200 million plant at the giant Lindsey Oil Refinery at North Killingholme, North Lincolnshire.
Unions claim Britons were not given any opportunity to apply for the posts.
For the Tories, shadow business secretary Kenneth Clarke condemned the strikes.
"I understand people being worried about their jobs. I don't think this is the right way to demonstrate it," he told the BBC Radio 4 Today programme.
Around 700 contractors at the Grangemouth oil refinery in central Scotland, who took unofficial action on Friday, walked out again today, although they also decided they would return to work tomorrow.
More than 1,000 demonstrators gathered for a mass meeting outside the Killingholme site and voted unanimously to allow union officials to start talks with management.
Kenny Ward, from Unite, told the crowd: "Over the last week your heroic actions here have inspired thousands in our county, hundreds of thousands in our country and millions across the globe.
"The fight started here at Lindsey - the fight against discrimination, the fight against victimisation and the fight to put bread on your table for your children. Gordon Brown said it is indefensible.
"If the Prime Minister will not defend the working man, if Parliament will not defend the working man, then the union will defend the working man."
Unite's Keith Gibson added: "I think there should be a call for industrial action right around this country to make the Government aware of how we feel and how we're not prepared to let this industry go to the dogs.
"I call on every trade unionist around this country in the construction industry to come out on official action."
Total later confirmed that talks will be held today between the firm and conciliation service Acas, senior union representatives and Jacobs, the main contractor at the Lindsey oil refinery.
The company said in a statement: "We recognise the concerns of contractors but we must stress that it has never been, and never will be, the policy of Total to discriminate against British companies or British workers. We have been a major local employer for 40 years and the majority of our 500 permanent staff are local. Our £200 million investment in the new HDS-3 unit will help to secure the long-term future of the refinery and local employment opportunities.
"We have a fair, competitive and rigorous tender process. We will continue to put contracts out to tender in the future and we are confident we will award further contracts to UK companies.
"We operate, and will continue to operate, under UK domestic law and the common European rules which apply to all UK companies operating elsewhere in Europe and European companies operating in the UK. Where sub-contracts are let, firms from both within the UK and elsewhere in the EU can bid for the work. Where we sub-contract, we do so on a fair and non-discriminatory basis; and the wage rates for this project are the same, or similar, as other equivalent jobs on the site."
Meanwhile engineering giant Alstom, which employs workers at the Staythorpe site and the Isle of Grain power station in Kent, said: "In line with both EU and UK employment laws Alstom never has, and never will, tolerate or instruct discriminatory action of any kind.
"This applies to all of its operations including engineering construction sites and includes for example, strict adherence to national agreements covering safe working, pay and conditions.
"British subcontractors and their workers on Alstom project sites in the UK will account for around two thirds of the total workforce employed to build a power station.
Alstom always gives all suitably qualified subcontractors from within the EU an equal chance to bid for work packages on its power station construction projects.
"Bids are carefully assessed and fairly awarded based on a range of industry recognised criteria which includes a company's track record on similar projects, the competitiveness of its commercial terms and proven evidence of technical expertise.
"Additionally, Alstom actively encourages its subcontractors to consider employing local workers in the event they require additional manpower."
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