Great Britain's former prime minister John Major and his wife Norma watch the 2016 Virgin Money London Marathon, London, April 24, 2016. Reuters/Paul Childs

Britain's close relationship with the United States would wither if voters opt to the leave the European Union in a June 23 referendum, Former Conservative Prime Minister John Major said on Friday.

"Our relationship with America would wither," Major told BBC radio. "America needs an ally inside the EU and it could no longer be us. And at home Scotland may choose to leave the UK."

U.S. President Barack Obama last week warned that Britain would be "in the back of the queue" for a trade deal with the United States if it left the EU.

Major, whose 1990-1997 premiership was plagued by disputes within his party over Europe, was the last Conservative prime minister before current incumbent David Cameron who is campaigning for Britain to remain in the EU.

"I'm no starry eyed Europhile but I have no doubt where our future lies," Major said.

He added that being part of the 28-member block has increased Britain's prosperity.

"Once the sick man of Europe we're now on course to be the biggest economy of Europe," Major said. "Painting Britain as a victim of Europe is simply ludicrous. We gain more than we give, we win more than we lose."