Former home secretary Alan Johnson will lead Labour's campaign to stay in the European Union.
Mr Johnson was appointed by acting Labour leader Harriet Harman to make a "persuasive" argument for the UK to retain ties with Brussels in the in/out referendum.
The appointment marks a return to frontline politics for Mr Johnson, who said the referendum would be the "most profound political decision of my lifetime".
Ms Harman said: "I am delighted Alan has agreed to lead Labour's 'Yes' campaign. The EU referendum is a hugely important moment for our country and our economy.
"We want this to be an outward-facing campaign that will listen to people's views and address their fears.
"There's no better person than Alan to argue for Britain to remain in a reformed EU in a persuasive way."
Mr Johnson has not held a front-bench post since 2011 when he quit as shadow chancellor, but is widely respected and regarded as one of the party's most effective communicators.
Explaining why he accepted the role he said: "The EU referendum will be the most profound political decision of my lifetime in terms of its effect on our national prosperity and Britain's position in the world.
"There is no such thing as a perfect institution but Britain must use its considerable influence to effect change, not lurk in the departure lounge.
"The Labour campaign that I lead will put the country's future above party machinations, emphasising the contribution that the EU has made to peace and prosperity across our continent."