The world's leading economies have warned that a British exit from the EU could cause a "shock" to the world economies.
Finance ministers from the G20 countries flagged the vote among key risks to world growth at a meeting in Shanghai.
Speaking after the meeting, the US Treasury Secretary Jack Lew said it was in the interests of the world for the UK to stay in the bloc.
Our view is that it's in the national security and economic security of the United Kingdom, of Europe and of the United States for the United Kingdom to stay in the European Union.
The British chancellor George Osborne told the meeting that there would be risks to jobs, living costs and living standards should voters opt to leave.
This isn't some adventurous journey into the unknown with all the humour attached to it. This is deadly serious.
It comes as David Cameron told voters in Northern Ireland that leaving the EU would be like taking a "leap in the dark".
The Prime Minister said this was because more than 60% of Northern Ireland's exports go to the EU and around 40% of its investments come from there.
Treasury figures put the number of jobs there at risk, which are linked to EU trade, at 50,000.
The UK will vote on whether to stay in the EU or leave in a referendum on June 23.
David Cameron has warned that Britain has "100 days to secure our future", as a poll suggests the final result hangs in the balance.
The EU referendum is a choice between "the devil and the deep blue sea", one of the UK's most senior business leaders is to say today.
Britain's car industry is an economic success story, and with uncertainty over the impact of Brexit, it wants to stay put.