The chance of the UK leaving the European Union (EU) is "just under 50%", according to Sir John Major.
The former Conservative Prime Minister, who spoke to Angela Merkel's Christian Democrat (CDU) party in Berlin, warned that Germany needs to agree to a series of reforms to keep Britain in the economic and political region.
"As the EU has moved from economic to political co-operation, that minority of malcontents has grown," Major said.
"In England, which is 85% of the population of the UK, opposition has reached a critical mass and now, for the first time, there is a serious possibility that our electorate could vote to leave the EU.
"I put the chance of exit at just under 50%. But if the negotiations go badly that percentage will rise."
"For the first time, there is a serious possibility that our electorate could vote to leave the EU," he said.
The former Conservative leader also argued that the UK's interests are best served inside the EU.
However, he reluctantly said that Britain could not "absorb" the number of immigrants coming to the country and reforms in Brussels were needed.
"I hate having to make this argument. I hate it. As a boy, I was brought up among immigrants in South London," he said.
"They were my friends and my neighbours. I have huge admiration for people prepared to uproot themselves to find work and a better way of life for themselves and their families. It takes a great deal of courage to do so. They deserve a warm welcome – not a chilly rebuff.
"I do not wish to close our doors to strangers – especially strangers with skills from countries that are often allies.
"But I do recognise – reluctantly – that our small island simply cannot absorb the present and projected numbers at the current speed: it is not physically or politically possible without huge public disquiet."
The speech comes after a landmark ruling from the European Court of Justice (ECJ) means that EU member states can stop so called "benefit tourism".
The decision, from the highest court in the EU, said countries within the political and economic union can refuse to give welfare payments to citizens who "solely" move to another member state to receive benefits.
The warning from Major also comes after the Office for National Statistics (ONS) revealed in May that net long-term migration to the UK was estimated to be 212,000 in the year ending December 2013 – more than the Conservative Party's net migration target of 100,000 by 2015.
The research body also explained that the majority of the arrivals (201,000) were from the EU, up from 158,000 on the previous year.