The Bulgarian President Rosen Plevneliev has warned David Cameron that his immigration policy risks turning Britain into a country of “isolation, nationalism and shot-term political approaches”.

Urging the Prime Minister to think on how he will be remembered by future generations, Mr Pleveliev added that his own country’s approach to its recent first wave of immigrants could be a “lesson” for the UK to follow.

His comments come as leaked Home Office plans for a 75,000 cap on immigration were criticised by the Lib Dem Deputy Prime Minister, Nick Clegg. He said Britain should not be “sticking a big no-entry sign on the cliffs of Dover”.

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And this morning Mr Clegg was backed by Business Secretary Vince Cable, his Lib Dem colleague, who told the BBC’s Andrew Marr show a cap would be “illegal and impossible to implement” and therefore “isn't going to happen”.

Speaking in an interview with the Observer, Mr Plevneliev asked: “Are we in Great Britain today writing a history of a switch to isolation, nationalism and short-term political approaches?”

In a direct challenge to the Prime Minister, he added: “Mr Cameron should address the daily agenda of British politics. But he should never forget that a politician is remembered in history not with the everyday business.

“Politicians should be ready to say the inconvenient truth and fight for unpleasant but necessary decisions which, in the short term, will bring our ratings down but, in the long term, preserve our values and keep the history of our proud tolerant nations as they are.”

Bulgaria recently experienced its first wave of immigration in modern times, amounting to some 11,000 people in the last two years, and its president said: “We might even be able to give a lesson to Great Britain. As a country that is not so rich and not so powerful, we are trying to understand not so much how many could come to Bulgaria but how we can integrate them.”

Mr Clegg also rejected an isolationist approach, writing in the Sunday Times that stopping immigration would “come at huge economic cost”.

He said: “What would happen if tonight every European living in the UK boarded a ship or plane and went home?

“Are we really that keen to see the back of German lawyers, Dutch accountants or Finnish engineers? Do we want the NHS to fall over and the City of London to grind to a halt?”

Estimates vary enormously on the number of Bulgarians and Romanians who will come to Britain when their EU migration restrictions are removed on 1 January.

Some thinktanks estimate as many as 50,000 will arrive every year, while the countries’ ambassadors predict a great deal fewer.

The Government is pushing through plans to make it harder for migrants to gain access to benefits, and while Downing Street accepts that freedom of movement is one of the founding principles of the EU, it says it cannot be unqualified.