Britain risks “alienating” friends in Europe by loose talk of migration curbs, the outgoing European Commission chief is to warn.

“It is an illusion to believe that space for dialogue can be created if the tone and substance of the arguments you put forward … offend fellow member states”, Jose Manuel Barroso is to say in a speech at the Chatham House think tank in London on Monday (20 October).

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“It would be an historic mistake if on these issues Britain were to continue to alienate its natural allies in Central and Eastern Europe, when you were one of the strongest advocates for their accession”.

The speech describes EU freedom of movement as a “basic right, which cannot be decoupled from other single market freedoms”.

It also urges the British government to back EU membership ahead of an in/out referendum in 2017.

“This case is one which needs to be made nationally. It is now high time to get out there and dispel the illusions”.

Barroso’s UK visit comes amid a shrill debate on EU immigration.

The ruling Conservative Party wants to renegotiate Britain’s obligation to the EU treaty principle amid electoral gains by the anti-immigrant Ukip party.

British media report one idea is to cap the number of net migrants to 100,000 a year by limiting national insurance numbers issued to unskilled workers.

The opposition Labour Party has also said it wants to impose English language tests before EU migrants can claim welfare.

Barroso in Chatham House is to challenge Ukip leader, Nigel Farage, who likes to pose on TV with a pint of beer in a symbolic defence of English traditions.

The speech says populist claims the EU is on a “relentless march to one single super-state” are false.

“I may prefer a glass or two of good red wine than a pint of beer when I am out on the election trail. But I too come from a country with a long history, a trading nation, proud of its culture and tradition … people living in Europe are also rather attached to their national identity”, Barroso will say.

He will note the UK would have less impact on climate change, the fight against Islamic State, and efforts to contain Russia if it did not act as part of the EU.

He made similar points in a BBC TV interview on Sunday.

He said he cannot comment on the 100,000 cap idea, but noted: “any kind of arbitrary cap seems to be not in conformity with Europeans laws".

He pointed out that 1.4 million Brits live in other EU states, including 700,000 in Gibraltar, who depend on free movement in Spain.

Referring to British PM David Cameron’s recent call for EU countries to set up Ebola screening, Barroso added: “What would be the influence [on the Ebola response] of the prime minister of Britain if he was not part of the European Union? His influence would be zero”.

Recent polls indicate Ukip could win up to 24 percent of votes in next year’s elections.

But British-friendly EU states, such as Poland, have, like Barroso, said there are limits to what they would do to keep the UK in.

“We want the UK to remain in the EU so we will do our best to help the British government introduce some reforms”, Poland’s ambassador to the UK, Witold Sobkow, told BBC radio at the weekend.

“[But] free movement of people is a fundamental freedom of the EU. So there are some things we can do and some things that we shouldn’t do”.