Lord Rose, former chief executive of Marks and Spencer, is to lead a cross-party campaign for the UK to stay in the European Union.
The Conservative peer, now chairman of the In Campaign, said the UK was "stronger in Europe" and to leave the EU would be a "leap into the unknown".
It comes after a group of Conservative, Labour and UKIP MPs launched a campaign to take the UK out of the EU on Friday.
David Cameron has promised a referendum on the issue by the end of 2017.
The prime minister held talks with German Chancellor Angela Merkel at Chequers on Friday, in which he stressed he wanted to "secure reforms and then keep the United Kingdom in a reformed EU".
A Downing Street spokesman said Mr Cameron had noted the talks were "progressing well" but there was still "much to be worked through in order to find a solution".
Lord Rose's appointment comes as a review of the "costs and benefits" of the UK's membership of the EU, to be conducted by the Treasury Select Committee, was announced.
Who is Lord Rose?
- Former management trainee for M&S
- Ex-chief executive for Burton Group, Argos and Arcadia Group
- Returned to the company in 2004 as chief executive, leaving in 2011
- Now chairman of online supermarket Ocado
- Knighted in 2008 for "services to the retail industry", and became a Tory peer in 2014
- Has advised the government on failing NHS trusts
The In Campaign - which officially launches its bid on Monday - said Lord Rose would take up the position with immediate effect.
Lord Rose said leaving the EU "risked" Britain's prosperity.
"The choice in the coming referendum is between remaining stronger, safer and better off inside Europe, or taking a leap into the unknown, risking our prosperity, threatening our safety, and diminishing our influence in the world," he said.
BBC political correspondent Robin Brant said the language used by Lord Rose was reminiscent of the successful Better Together campaign during last year's referendum on Scottish independence.
The campaign could play on "the fear factor" of the UK leaving the EU, said our correspondent, adding: "He [Lord Rose] talks about a 'hard-headed practical decision', the UK being 'safer and stronger'.
"But also he talks about 'a leap into the unknown', and I think that is something that is similar to the message that came from the 'No' campaign."
EU referendum: What you need to know
Will Straw, executive director of the In Campaign, said Lord Rose brought "unparalleled experience of business and enterprise".
"He knows the value that Britain's membership of the EU brings to businesses right across the UK, and what would be at risk if we left," he said.
Vote Leave, made up of Conservative, Labour and UKIP MPs and donors, launched its campaign on Friday.
Supporters of the campaign, which says it wants to negotiate a new deal based on free trade and friendly co-operation, include Labour's Kate Hoey and UKIP's Douglas Carswell.
It is competing with a rival group - the UKIP-backed Leave.EU - to be the official campaign to leave the EU.
'Remain or leave?'
The Electoral Commission has yet to designate the official campaigns on either side of the EU debate.
The chosen groups will benefit from increased spending limits of £7m during the campaign period, campaign broadcasts, and a free mailout to households.
They all get access to public meeting rooms and to the electoral register, and are entitled to public grants of up to £600,000.
The Electoral Commission has recommended voters in the election are asked whether the UK should "remain in" or "leave" the EU, rather than being asked to answer a "yes or no" question.
The government has tabled an amendment to the bill.