Britain's biggest civil service union said on Wednesday its members would join teachers and stage industrial action later this month, meaning some 750,000 public sector workers will strike over the government's pension reforms.
Mark Serwotka, general secretary of the Public and Commercial Services (PCS) union, also warned that unless the government changed its plans for public sector pensions, millions of workers could take action later this year.
Unions, angry at the government's massive spending cuts to address a budget deficit of around 10 percent of GDP, have vowed to stage coordinated national action in what would be Britain's worst labour stoppages for decades.
"The ballot mandate from our members is that we will take national strikes with other unions and we believe this will be the first of a number of those," Serwotka told reporters.
The PCS said 250,000 workers had voted to hold a 24-hour walkout on June 30, joining two big teaching unions which announced on Tuesday that members had voted for a strike.
"Schools will be shut, job centres closed, driving licences won't be issued, queues will form at airports and ports," Serwotka said. "There will not be any person in the UK that will not see the effectiveness of this action."
The row centres on plans to reform public sector pensions, which the government say have become financially unsustainable because people are living longer.
Ministers argue they are also unfair to private sector workers, who retire later and receive lower benefits.
Unions say the plans will mean people will have to work longer and pay more for worse pensions, which will be based on average career earnings rather than final salaries.
The move also comes on top of widespread public sector job losses and a two-year pay freeze.
Dave Prentis, head of the Unison union, has warned about 1.2 million of his members in schools, councils and the health service could down tools later this year and Serwotka said they were discussing action with Britain's largest union Unite.
"The government should understand that this is going to grow and grow. None of us want to be here in October talking about four million people on strike," Serwotka said.
The coalition government has said it is putting contingency plans together to deal with any widespread strikes and has hinted that it would consider tougher new union laws.
Cabinet Office minister Francis Maude, who is leading negotiations with unions, said there was limited support for the PCS strike among its members, with less than 20 percent voting for the action.
"There is no justification for any civil servant going on strikes while discussions are continuing," he said in a statement.
"There is cross party consensus that public sector pensions need to be made fairer and more sustainable. Public sector pensions will remain among the very best, providing a guaranteed pension level for all employees."