One of the UK's biggest trade unions has written to its members to urge them to vote for Labour in June's European elections, it has announced.
Elections to the European Parliament take place in June
Amicus, which represents more than a million workers, says Labour is the only party capable of making a real difference for working people.
Success for the Conservatives would damage the economy, it warned.
The move is a boost for Labour after criticism from other unions put a strain on its relationship with them.
Amicus, which represents professionals working in the health, manufacturing, science and finance sectors, is the UK's largest private sector union. It was formed in 2002 when the MSF and AEEU unions merged.
The union's assistant general secretary Danny Carrigan said it wanted to show constructive support for Labour and warned against deserting the party.
"The right-wing press is telling us that at last there is competition in elections. We couldn't agree more," he said.
"The choice is progress with Labour or Maggie's mates back in the driving seat."
European elections take place on 10 June and will be the first since the European Union was enlarged from 15 to 25 member states.
Only 24% of the UK electorate voted last time Europe went to the polls, in 1999 - the country's lowest turnout for a European election, and around half the percentage that turned out across the EU as a whole.