Poland has threatened to veto a key plank of David Cameron's planned European Union reforms - in retaliation for claims its citizens are exploiting benefits rules.
The row threatens to derail the Prime Minister's hopes of renegotiating the UK's relationship with the EU before an in/out referendum in 2017.
In a double blow to the Conservative leader, the Coalition rift over immigration also appeared to deepen yesterday.
Deputy Prime Minister Nick Clegg backed the Liberal Democrat's Business Secretary Vince Cable, who had delievered a warning that the Coalition would fail to reach its target to cut migrants numbers to tens of thousands.
Mr Cameron faces a tense discussion with his Polish counterpart today.
Polish Prime Minister Donald Tusk accused the Prime Minister of singling out Poland amid warnings migrants should not receive child benefit for youngsters living in their home country.
Mr Tusk said: "We will not agree to it if these are changes that would stigmatise any particular national minority.
"Nobody has the right to single out Poles as a particular group that abuses or exploits something."
Downing Street sources denied that Mr Cameron was wrong to single out Poland and said that every country would "have its position" on potential treaty changes.
The Home Office has said that it is committed to cutting net migration to the UK to the tens of thousands.
But the latest figures show that the numbers increased last year.
Mr Clegg said: "I think the Business Secretary was entirely right in pointing out that Government needs to be open with the British people
"He quite rightly pointed out that the number of British people leaving Britain to go elsewhere, indeed those Brits living elsewhere and coming back, is something that no Government can necessarily control."