Protesters turned out in Lincolnshire despite the extreme weather
Contractors at two nuclear plants have walked out in support of protests over the use of foreign labour, in the latest of a wave of unofficial strikes.
The walk-outs at Sellafield and Heysham came on the day talks began over the dispute, which started at Total-owned Lindsey oil refinery, in Lincolnshire.
Workers at Grangemouth oil refinery and power stations in Longannet, Warrington and Staythorpe have also walked out.
Total insists it is not discriminating against British workers.
A statement said: "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."
The mediation service ACAS met Total managers, its main contractor Jacobs, and union leaders for talks in Scunthorpe on Monday.
The talks were adjourned and will resume on Tuesday after agreeing the terms of reference, involving studying the contract at the heart of the row.
Thousands went on strike last week in a series of unofficial walk-outs around the UK, to show solidarity with workers protesting at the use of Italian and Portuguese labour at the Lindsey refinery in North Lincolnshire.
An unofficial strike is a counterproductive way of solving problems that can actually be solved by discussion and by negotiation
Workers there were angry a contract to expand the refinery was sub-contracted by Jacobs to an Italian firm, IREM, which decided to use its own workforce.
Business Secretary Lord Mandelson said he expected Acas to review the situation at the refinery "very quickly" and urged striking workers to go back to work while this process was taking place.
In a parliamentary statement, he said he believed the refinery operator had not discriminated against UK workers, which he stressed would be illegal under European law.
He added the government was committed to the "robust enforcement of employment rights" and "proper application of EU rules" which, he said, were vital to supporting the UK's substantial trading relationship with the EU.
But protests gathered pace on Monday:
• About 300 protesters gathered at the Lindsey refinery's terminal gates.
• Around 600 workers met in a car park at Sellafield, in Cumbria, to discuss industrial action and about 1,300 workers are believed to be taking part in a 24-hour stoppage at the plant.
Strike organisers at Lindsey oil refinery address workers
• About 400 contractors at Longannet in Fife voted to stay out for 24 hours, and to return for another meeting at 0730 GMT on Tuesday.
• Some 300 contractors at the Grangemouth oil refinery in central Scotland walked out but decided they would return to work on Tuesday.
• Around 200 construction workers at Fiddlers Ferry Power station, near Warrington, Cheshire, have again downed tools, following similar action on Friday.
• About 150 contractors walked out at Drax Power Station, near Selby in North Yorkshire.
• The owners of Coryton oil refinery in Thurrock, Essex, said a number of workers had walked out but refinery operations were unaffected.
• Some 500 contractors at the South Hook LNG terminal, in Milford Haven, west Wales, downed tools.
• Fifty contract workers from Didcot power station in Oxfordshire have downed tools, but there is no disruption to power output.
Sellafield striker and GMB convener Willie Doggert said: "All we want is a level playing field - it's not just about foreign workers, we need jobs to be advertised with transparency - so that everybody gets a far crack of the whip."
The Unite union has proposed a three-point plan for dealing with the walk-outs.
Joint leader Derek Simpson said the immediate problem should be tackled and an investigation launched into the practices of contractors.
He also said moves should be taken to overturn European legal precedents, which he said allowed employers to undercut wages and conditions.
Prime Minister Gordon Brown said he understood fears over jobs were behind the walk-outs.
"But I also say that an unofficial strike is a counter-productive way of solving problems that can actually be solved by discussion and by negotiation," he said.
The government has said it might challenge EU law to stop cheap foreign labour "undercutting" British workers.
Total said it operated under UK and EU laws, that it sub-contracted "on a non-discriminatory basis" and that wage rates were the same as those offered for equivalent jobs on site.
The European Union said it had "sympathy" with workers, but "the internal market is actually our best platform to maintain a high level of employment".
A spokesman for the European Commission said: "All the evidence from past crises shows that the moment you enter a spiral of closing borders to each other, at the end of the day all will be the poorer and will have less employment."
Shadow business secretary Kenneth Clarke said the strikes were not the "right way" for people to demonstrate their concerns.
Nigel Farage, leader of the UK Independence Party, which supports Britain's exit from the EU, said: "'British jobs for British workers' will only happen when Britain is run by and for Britons."
But the Lib Dems have warned against any move by the government to exempt Britain from EU employment laws.
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