Sir John Major has warned the UK would face a "lesser future" if it leaves the European Union as he urged continental leaders to accept changes are needed to ease pressures caused by the number of migrants coming to Britain.
The former prime minister said yesterday that the country's membership of the organisation is in the balance.
He urged other European states to agree to measures to help curb the number of migrants from within the union.
Sir John said the issue is not a "uniquely British problem", and he is confident a solution could be found despite leaders including German Chancellor Angela Merkel ruling out changes to the principle of free movement within the EU.
Sir John, who addressed members of the German chancellor's party in Berlin last week, said there had been a "huge bulge" in numbers coming to the UK, but the problem "may only be relatively short-term".
Warning of the consequences of a British exit from the EU, Sir John said: "Of course there would be a future. But it would be a lesser future."
He added: "I really would not want to be the prime minister who had to explain that we are sinking to a much lower level of relevance in the world outside the EU, with the doors in the corridors of power being closed to us.
"On every count, despite its frustrations - of which there are many, despite the reforms we need - which are many, we are far better off in the European Union than outside it and, most important of all, we are far better off for the next generation and the generation after that if we are in."
Sir John said the UK's word would mean less and its economic power would be "materially decreased" outside the EU.
"Britain has been a great nation in the last 300 years. Do we really want to sink to a lower level of relevance outside the European Union?"
David Cameron has promised to put immigration at the heart of his plan to renegotiate the UK's relationship with Brussels before an in/out referendum by the end of 2017 if he remains in Number 10.
Sir John told BBC1's Andrew Marr Show: "We aren't seeking to end free movement, but what has been happening over the last few years has been such a huge bulge in the amount of migrants coming to the UK - our population has risen by about seven per cent in a decade and at the present rate the British population would rise in a few decades by 25 per cent while the German population would have fallen.
"I think as people begin to see the particular circumstances that we face I think there will be a good deal of sympathy for the difficulty, and the European Union has a good deal of finding a way around difficult corners like this."
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