Mr Cameron flew into Warsaw for crunch talks with new prime minister Beata Szydlo (pictured) amid growing signs that Poland could block his plan

David Cameron was fighting to save his EU reform package last night after Poland warned it could not accept his bid to curb benefits for migrants.

Mr Cameron flew into Warsaw for crunch talks with new prime minister Beata Szydlo amid growing signs that Poland could block his plan to ban EU migrants from claiming benefits for their first four years in the UK.

Speaking ahead of a working dinner yesterday, Mrs Szydlo described the proposal as unacceptable.

Poland’s foreign minister Witold Waszczykowski confirmed the depth of the rift, telling a Polish radio station: ‘In the case of (migrant benefits) we have a large dispute with David Cameron. He wants to limit one of the basic EU treaty freedoms – free movement of citizens.’

Mr Cameron also faced a frosty reception on the issue in Bucharest yesterday where Romanian president Klaus Iohannis pointedly told him that tens of thousands of Romanians working in Britain ‘pay tax’.

Both Poland and Romania want the UK to remain in the EU, but fear they would face a backlash from their own voters if they agreed to reforms that would ‘discriminate’ against their citizens.

Mr Cameron said in Bucharest that he supported the basic EU right of free movement, but warned that current net migration levels to the UK of more than 300,000 a year were unsustainable.

He added: ‘We do need to find ways to allow member states to make changes to their social security systems that will help them to deal with this issue.’ In a further sign of the difficulties engulfing the Prime Minister’s flagship demand, Brussels has now tabled compromise proposals to introduce an ‘emergency brake’ to allow states struggling with immigration to temporarily impose restrictions.

European Council president Donald Tusk warned Mr Cameron this week that there was no consensus within the EU on the UK plan to curb migrant benefits, which is the only element of the reform package designed to tackle public concern about immigration.

Yesterday Mr Tusk said Mr Cameron will ‘not be satisfied 100 per cent’ when the deal is finally agreed. The former Polish prime minister, who chairs meetings of all 28 EU leaders, said the four-year migrant benefit ban was ‘hugely problematic’.

Romanian Prime Minister Dacian Ciolos (right) and British Prime Minister David Cameron (left) chat during their official meeting held at Victoria Palace, in Bucharest yesterday

Diplomats in Brussels are now working on a ‘Plan B’ that would let member states impose temporary restrictions on migrant numbers if they can show that immigration is placing key services such as health, education and housing under acute strain. But it appeared that Brussels would have the final say over if and when Britain could apply the ‘emergency brake’, and how long it would last.

Downing Street declined to comment in detail on the deal. But government sources indicated they remain to be convinced alternative proposals for curbing migration from the EU would have a significant impact.