New Zealand's prime minister has insisted Britain is stronger as part of the European Union in an intervention David Cameron labelled "important".
John Key said his country would be looking to join the EU rather than leave it if the trading bloc was on the Pacific nation's doorstep.
Mr Key, a friend of Mr Cameron's, spoke out as he met the Prime Minister on the margins of a nuclear security summit in Washington.
"We certainly think it's a stronger position for Britain to be in Europe," Mr Key said.
"We see Europe as an extremely important continent that needs strong leadership. We think Britain provides that leadership.
"We are currently going through the process of wanting to sign a free trade agreement with Europe because it is the home of 500 million people and a huge amount of middle income consumers.
"Both David Cameron and Angela Merkel have been the leaders of looking to get New Zealand into a free trade agreement.
"As someone who has lived in the United Kingdom for a long period of my working life, I guess all I would say is that if we had the equivalent of Europe on our doorstep, New Zealand as a country would be looking to join that. We certainly wouldn't be looking to leave it.
"But the British people will make up their own minds what they think is appropriate and they will vote accordingly."
Mr Cameron said the comments made by his New Zealand counterpart were "important".
Will Straw, executive director of Britain Stronger In Europe, said: "John Key has underlined the vital importance to British consumers and businesses of the UK's access to Europe's single market, which is the best free trade deal available to the UK.
"Of everything we sell to the rest of the world, almost half is sold to Europe: it's our home market. Walking away would mean cutting ourselves off from access to the EU's 500 million consumers and all of the economic opportunities that brings us.
"Our membership of the EU gives us clout across the world and thanks to our membership we benefit from free trade agreements with more than 50 countries. John Key himself makes clear that Britain is stronger in the EU than we would be out on our own. Leaving the EU would risk our prosperity, threaten our safety and diminish our influence in the world."