British consumers will suffer if the UK votes to leave the European Union, the former boss of Marks & Spencer has told ITV News.
Lord Rose, who is leading the campaign to keep the UK in the EU, estimates that a British exit from the single market would cost businesses and households an extra £11 billion a year.
A Brexit will have a "direct implication on everybody's ability to spend money", he told Joel Hills.
Rose, who runs Britain Stronger in Europe, said employers' ability to keep people in work and to "keep the economy moving forward" would also be put in jeopardy.
The businessman was speaking to ITV News as David Cameron met his European counterparts in Brussels to discuss his plans for EU reform.
One of the prime minister's main aims is to limit the amount of benefits migrants can claim in the UK.
Rose said while he understood voters' concerns over the number of EU immigrants, without an influx of "hardworking" people "we would not have a country that is flourishing today".
"We would actually come to a grinding halt," he insisted.
He said Britain currently had the "best of both worlds" in its relationship with the EU as it was engaged in an "economic trading partnership" without being in the euro.