Why can't British jobs be saved for British workers, as the Prime Minister suggests?
Since Britain joined the European Community in 1973, the principle of free movement of labour across borders was established. But it was not until 2004 that this became an issue, when a number of poor eastern European countries joined the EU, with workers attracted to the UK's higher salaries. Britain and Ireland lifted all restrictions in 2007, leading to the influx of Polish people in particular.
Thousands strike over migrant workers: 'British jobs' slogan backfires on Brown in UK-wide action
Around 1,000 contractors – many holding placards bearing the Prime Minister's controversial phrase – also gathered at the Lindsey oil refinery at the heart of the dispute, in North Killingholme, Lincolnshire.
Protests began at the site on Wednesday after the French-owned Total plant announced a £200 million construction project had been awarded to an Italian firm, which was bringing in 400 Italian and Portuguese staff to complete the work.
Yesterday there were also protests in Wales and Northern Ireland, with the threat of the unofficial strikes spreading on Monday to the Sellafield nuclear plant in Cumbria.
Britain's membership of the European Union allows workers to take jobs in any of the 27 member states, but anger had been sparked at the Total plant by the belief that local unemployed engineers had lost out to cheaper immigrant workers.
Mr Brown, speaking from the World Economic Forum in Davos, Switzerland, said he understood the concern about jobs, but called for countries not to retreat into "protectionism".
But in London, the Prime Minister's spokesman risked inflaming the situation. Asked whether Mr Brown now regretted his "British jobs for British workers" phrase – first made just before he became Prime Minister in 2007 – the spokesman said: "I don't see a reason for regret."
While admitting there were no restrictions on EU workers in the UK, he said the free market allowed British firms to compete on an equal basis on the Continent with European rivals.
The spokesman said Total had awarded the contract at a time when there was a shortage of skilled UK engineers.
Five UK and two European firms tendered for the work, which was awarded to the Italian company Irem on the basis that it was supplying a permanent workforce.
Downing Street is planning to speak to the parties involved in the dispute next week, but the spokesman emphasised the situation was for "workers to resolve with employers".
Tim Finch, a migration expert at the Institute for Public Policy Research think-tank, said: "'British jobs for British workers' was a careless slogan that is coming back to haunt the Prime Minister. With free movement of labour inside the European Union, jobs cannot be ring-fenced."
Bobby Buirds, the regional officer for trade union Unite in Scotland, said: "The argument is not against foreign workers, it's against foreign companies discriminating against British labour. This is a fight for work. It is a fight for the right to work in our own country. It is not a racist argument at all."
Companies affected by the wildcat action said front-line operations were unaffected as most of the staff who walked out were employed by sub-contractors.
Around 700 staff stopped work at Grangemouth, but BP said the Forties pipeline to the North Sea was unaffected. About 500 walked out at ScottishPower's Longannet power station, and 100 at its Cockenzie plant, while 80 stopped work at British Energy's Torness nuclear power station.
First Minister Alex Salmond urged workers to return to work once they had made their point.
• Gordon Brown delivered a plea for Britons to remain optimistic in the face of the financial crisis last night. Speaking from Davos, he urged against "talking the country down".
Q & A
What is the dispute about?
Unions are concerned that engineers, who tend to travel the country on fixed-term contracts, will lose job opportunities as a result of contracts being won by foreign firms. At the Total refinery in Lincolnshire, Jacobs, a US-based company, won a construction contract in 2006. It then sub-contracted some work, and an Italian firm, Irem, won, beating bids from five UK firms. It has decided to use its Italian and Portuguese staff to do the work. This has angered locals at a time when unemployment is rising.
Will it affect petrol supplies or risk power cuts?
Not in the short term. BP, which is responsible for the Forties pipeline at Grangemouth which links to the North Sea oil fields, said 100 workers employed by a sub-contractor had taken action. A BP spokesman said: "The workers are not involved in day-to-day operations and the Forties pipeline system will continue to operate as normal." INEOS, the main operator of Grangemouth, said: "There will be no impact on production levels at all."
How many foreign workers has Total employed through the Italian firm?
A total of about 100 Italian and Portuguese workers are on site. They are expected to be joined by 300 more next month.
Is the Scottish Government getting involved?
Alex Salmond, the First Minister, urged staff to return to work. He said: "At times like these, it is really important that nobody does anything that jeopardises any jobs whatsoever."
Why can't British jobs be saved for British workers, as the Prime Minister suggests?
Since Britain joined the European Community in 1973, the principle of free movement of labour across borders was established. But it was not until 2004 that this became an issue, when a number of poor eastern European countries joined the EU, with workers attracted to the UK's higher salaries. Britain and Ireland lifted all restrictions in 2007, leading to the influx of Polish people in particular.