The UK will face a "lesser future" if it leaves the European Union, Sir John Major warned as he urged leaders on the continent to accept changes are needed to ease the pressures caused by the number of migrants coming to Britain.
The former prime minister, who has warned the UK's membership of the EU is in the balance, urged other European states to agree to measures to help curb the number of migrants from within the union.
He 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 7% in a decade and at the present rate the British population would rise in a few decades by 25% 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."
The former premier insisted he is not anti-immigration, saying "we wouldn't have a National Health Service without migrants, we would not have a transport system without migrants", but the problem is "purely numbers, and it may only be relatively short-term".
He added: "I see it as a shortish-term problem, maybe not a year, maybe longer, and we need a little help over that period."
Sir John said he believes a way can be found to make changes without altering the fundamental principle of free movement.
"I think there are some practical things that could be done that don't infringe the principle but do meet the problem," he said.
The former Tory chief said Mrs Merkel is not the only leader opposed to changes in free movement rules, but there are other countries who share the UK's concerns.
"It's not just Angela Merkel, of course she is hugely important... but it is an agreement we have to have across the European Union.
"It's not only we who face trouble. Many of the far-right parties, many of the anti-social parties who offer nothing but negativity across Europe - in Greece, in Sweden, in many countries including our own - have an antipathy to immigration because immigration is seen by their populations at too high a level, as causing difficulties.
"It's not just a uniquely British problem. It's uniquely difficult for us, because of the numbers and because we, for example, are a fraction of the landspace of France or Germany."
Following his recent return to the political fray on the issue, Sir John was asked if he would be prepared to help lead the negotiations.
"I think not, I think it will have to be someone who is in the Government and is close to the Prime Minister in the Government, but a great deal of the most important negotiations... will inevitably be done by the Prime Minister himself."
He disagreed with Foreign Secretary Philip Hammond's assessment that the UK has to be prepared to walk away from the table if a deal cannot be reached.
"I don't think it's a question of saying we would walk away if we fail because I don't anticipate failure," he said.
Sir John's intervention came as the Government faced questions about its handling of European Union budget negotiations amid allegations that Chancellor George Osborne struck a secret deal over the UK's contributions to Brussels.
Despite previously opposing proposals to boost the EU's contingency fund for this year, Britain abstained in a vote that would have handed an extra £2.4 billion to the European Commission, the Sunday Times reported.
The move led to claims - strenuously denied by the Treasury - that the UK agreed not to oppose the move as part of the effort to reduce the surprise £1.7 billion surcharge demanded from the UK by Brussels last month.