Alan Johnson, the former Labour cabinet minister, has agreed to head the Labour yes campaign during the referendum on Britain’s membership of the European Union, Harriet Harman, the Labour interim leader, has told the Guardian.

Johnson, repeatedly courted by Ed Miliband and others to return to frontline politics, has agreed to head up the campaign which will be separate to any cross party pro-European organisation.

Seen as one of Labour’s most persuasive communicators, he is regarded as the right man to carry a Labour pro-European message which would not leave a permanent rupture with the thousands of Ukip supporters that Labour needs to win back ahead of the 2020 election.

His appointment suggests Labour is to run a full throated pro-European campaign, despite some shadow cabinet members previously hintingthat it might be advisable to take a low profile in an attempt to woo back those lost Ukip voters.

Johnson told the Guardian: “This is not a return to the shadow cabinet, but Harriet is always very persuasive. There will be a lot of different kinds of campaigns for a yes vote, but I don’t think there has ever been an argument that Labour should not have its own campaign. As a political party we have to be very clear where we stand on this issue. I see this as the most important political decision of my lifetime.

“It’s more important than the referendum decision in 1975 which almost felt like a decision to go in. I know we had been in the EU with six member states for a few years. But it would be much more momentous to leave an institution that is now 28 members strong and in which we have 40 years’ active membership, all at a time when the world is more interdependent. It would be a much more important and profound decision to leave, having been in.”

Harman said: “This is a big moment of choice and he is prepared to play his part. Alan is well known. He is a gifted communicator. He is unequivocal in his pro-European views. It is part of his DNA. He reaches all parts of the country.”

With his background as a former union general secretary and business secretary, he warned David Cameron not to try to interfere with existing social and employment rights, including the working time directive, as part of his renegotiation.

His warning was backed up by a statement on Thursday from the GMB general secretary Paul Kenny saying: “If what David Cameron brings back from the re-negotiations tilts the balance even further away from standards for workers, as the CBI wants, many organisations traditionally in favour will campaign for a no vote.”

Johnson said: “It would be very difficult for Cameron to bargain those rights away away. I don’t think other member states would wear that. He needs unanimity. People did not have the right to a day off until the working time directive came in.”

He added: “I also think there is only one person in Britain who wants a yes vote more than me, and this is David Cameron. When you are the prime minister, you see the full horror of Britain leaving the EU, and then it is no longer about rhetoric, debating societies, and party cliques. Of course he wants to stay in. So he would be mad quite frankly to put at risk the support of millions of people dependent on those social rights. If he bargained those rights away he might fill one hole in, but then he will only have dug a deeper one, so my advice would be not to go there, but he probably knows that.”

He also insisted the party had to be straightforward in its pro-Europeanism. “My clear view is that we do not play political games on this. Our policy and outlook is pro-European, so I am not interested in the game of saying one thing to people in the North because we have got Ukip issues there and then something else in the Midlands.”

He said he doubted that every single person who voted Ukip wanted to leave the EU any more than every Labour voter supported the mansion tax. He said that what Ukip had traded on was immigration, and that he had met plenty of people who had issues about free movement of labour but accepted it as the price for being in the EU.

“The focus has to be on growth and jobs and the terrible damage it would do to the UK economy if we withdrew. Once people focus on the risk of leaving the EU I think it will be a yes vote.”

He denied that his decision to take the job made a permanent return from rank politics saying this was a job “until the end of 2017 and no more”.

Harman said the reasons that made some voters desert Labour for Ukip were not all about Europe, but broader issues. She said: “We are going to be very clear and straightforward in our arguments. There is not going to be any triangulation and Alan is not a triangulator.”