A declaration by the Group of Seven says a Brexit will pose a real threat to the global economy.
Leaders voiced their concern at a summit in Japan.
There is a hope within the world’s capitals that Britain remains in the EU, but it is up to the British electorate to decide.
The British Prime Minister David Cameron, addressing the media in Japan, said: “This is not a bunch of sort of academic forecasters sitting in a room chewing the end of a biro and coming up with a number. If you want to leave the Single Market you are going to get worse access to it. If you get worse access to it it is bound to affect your economy. And from there everything else follows.”
The announcement comes following IMF comments that there are no economic positives to Britain leaving the EU at time when the global economy is crying out for growth.