Commenting on the OECD's forecast that UK growth will slow to 1% next year, Chuka Umunna MP, leading supporter of the Open Britain campaign, said:
"The OECD has made clear that the vote for Brexit has had a negative impact on our economy. Britain is facing a darker economic future.
"But our future will be poorer still if we abandon the Single Market, our biggest single customer in the world.
"Europe remains vital to Britain's long-term prosperity. For business, investment and jobs, it is crucial that we remain a member of the world's largest market."