The campaign to keep the UK inside the EU has seen its left-wing credentials boosted once more after the Communication Workers Union (CWU) voted to back a 'remain' vote at the 23 June referendum.
The decision from the 197,000-strong union, at its annual conference in Bournemouth, means the CWU joins Unite, Unison, the Trades Union Congress and the GMB in opposing a Brexit.
Alan Johnson, a former general secretary of the union, said he welcomed the "overwhelming" vote for 'remain'. "As a former postman and CWU General Secretary, I know the vital difference that union members can make to this campaign," the Labour In for Britain chair added.
"The decision by CWU members highlights the unity of the Labour movement with all the biggest affiliated unions now campaigning for Britain to remain in the EU. Labour is united in campaigning for Britain to remain in Europe because of the jobs, growth and investment and protections for British workers and consumers that depend on our EU membership."
The decision comes ahead of Jeremy Corbyn's speech to the CWU conference. The Labour leader is also campaigning for a 'remain' vote at the historic referendum, but the British labour movement is not entirely united on the issue. Transport unions Aslef and the RMT are campaigning for a Brexit, as well as the Bakers' Union.
"EU policies are at odds with the aspirations of this union as the various treaties and directives are demanding the privatisation of our rail and ferry industries," said Mick Cash, the general secretary of the RMT.
"The EU is also secretly negotiating trade deals [TTIP] with the US and Canada that will decimate our health and education sectors and hand huge powers to transnational corporations over nation states and their governments.
"The Tories will be campaigning to stay in the EU come any referendum as they support this right wing, neo-liberal, anti-worker agenda."
Latest EU referendum opinion polls
|16-18 Apr||Ipsos MORI||49%||39%||8%||Telephone|