Gordon Brown today condemned wildcat strikes as "indefensible" amid efforts to prevent the row over foreign labour escalating into mass industrial action.
The prime minister said he recognised that people were worried about jobs being taken by workers from other countries, but stressed that the UK was part of a single European market.
In an interview for the BBC's Politics Show, he sought to explain his pledge of "British jobs for British workers", insisting he had only meant people would be given the skills to compete against other nationalities.
Asked for his message to those thinking of staging sympathy strikes tomorrow, Brown said: "That that's not the right thing to do and it's not defensible."
Officials from government departments, unions, employers and the mediation service Acas have been in discussions after a series of wildcat strikes across the country on Friday.
The protests were prompted by a decision to bring in hundreds of Italian and Portuguese contractors to work on a new £200m plant at the Lindsey oil refinery, in North Lincolnshire. Unions claim Britons were not given any opportunity to apply for the posts.
The justice secretary, Jack Straw, appeared to suggest some sympathy for the protesters, saying it was important to establish whether "British firms and British workers had a fair crack of the whip" in bidding for the contract.
He said Brown had been "very clear" what he meant when he promised British jobs for British workers.
"The last person in the world, and everybody knows that, to be talking about protectionism is the prime minister," Straw told Sky News. "What he was talking about was ensuring that we build up the skills base in this country so when a firm wants to take someone on, they don't have to look abroad."
The health secretary, Alan Johnson, a former trade union leader, said wildcat strikes were unhelpful but he understood the anger. He said free movement of labour was fundamental to the EU, but suggested European court judgments could have "distorted" the rules.
"If workers are being brought across here on worse terms and conditions to actually get jobs in front of British workers on the basis of dumbing down the terms and conditions, that would be wrong and I understand the anger about that," Johnson told the BBC's Andrew Marr Show.
"These various judgments have distorted the original intention and we need to bring in fresh directives to make it absolutely clear that people cannot be undercut in this way."
Yesterday, the business secretary, Lord Mandelson, promised the government would "make sure that both domestic UK law and European rules are being applied properly and fairly.
"But it would be a huge mistake to retreat from a policy where within the rules, UK companies can operate in Europe and European companies can operate here," Mandelson said. "Protectionism would be a sure-fire way of turning recession into depression."
The government's stance was given short shrift by unions. Paul Kenny, general secretary of the GMB, said: "No company should be able to discriminate against anyone on the grounds of where they were born.
"You simply cannot say that only Italians can apply for jobs as has happened in this case. No one is saying that different countries cannot bid for different contracts. What is happening here would be illegal under UK domestic law."
British Prime Minister Gordon Brown has condemned the workers who staged mass walkout across the country on Friday.
File photo of Gordon Brown 
Speaking to the BBC from the World Economic Forum in the Swiss skiing resort of Davos, he said that the walkouts were "not the right thing to do". Workers on industrial sites in Wales, Scotland, Cheshire, Hampshire and Teesside had left their plants and staged protests outside in sympathy with staff at Lindsey Oil Refinery in Lincolnshire, after French owner Total had given a contract to an Italian firm. The firm brought 300 Italian and Portuguese staff into the plant.

Brown had made a speech in 2007 where he called for "British jobs for British workers". He explained to the BBC's Politics Show that "when I talked about British jobs, I was taking about giving people in Britain the skills, so that they have the ability to get jobs which were at present going to people from abroad". The government says that European Union (EU) law makes it impossible for them to provide British jobs for British workers in the EU common market. Earlier, the Secretary of State for Health, Alan Johnson, told the BBC that recent European Court of Justice judgments had undermined employment rights but that the government would be pushing the EU to take action if it finds that the foreign workers are 'undercutting' UK citizens. He described the walkouts as "unhelpful", but said he understood the anger over EU workers coming to the UK "on worse terms and conditions to actually get jobs in front of British workers on the basis of dumbing down the terms and conditions." 

The walkouts were also condemned by the opposition Conservative Party. Foreign affairs spokesman William Hague said that "strikes are never the way forward" and that his party strongly supported the free market in labour in the EU. He said that it was "so unbelievably ridiculous and silly" for Brown to have made the promise on British jobs in the first place.

The government has asked the Advisory, Conciliation and Arbitration Service (ACAS) to examine the circumstances behind the strikes and see if British workers are being unlawfully excluded, as the GMB union has claimed.

Secondary or sympathetic industrial action has been illegal in the UK since the 1980s. However, the penalties apply only to unions organizing secondary action and not to individual workers walking out.