British Prime Minister Gordon Brown has condemned nationwide wildcat strikes over the use of foreign workers at a time of soaring unemployment, as unions warned more staff may down tools this week.
Industrial unrest is spreading across Europe, fuelled by a worsening economic downturn. Last week, French workers protested for more pay and job security while Greek farmers set up roadblocks, demanding compensation for low prices.
Mr Brown, asked by BBC Television about last week's walkouts at energy plants across Britain, said they were not defensible, and "not the right thing to do."
The government has asked an independent mediator to look at whether skilled British workers were unfairly debarred from contract work at a refinery in eastern England owned by France's Total. Thousands across Britain protested against the use of Portuguese and Italian workers at the site.
"Whether you call it indefensible or not, a lot of people find it understandable in the circumstances," said Derek Simpson, joint chief of Britain's biggest union Unite.
Unions have warned the strikes will continue this week and may even escalate as more workers across Britain discuss whether to join the action.
Unemployment is rising sharply in Britain, up by 131,000 in the three months to November to 1.923 million. The rate of joblessness hit 6.1% - the highest since 1999 - and more big rises are expected as the downturn gathers pace.