Poland, Hungary, Slovakia and the Czech Republic have issued a statement rejecting any British demands of the European Union that are discriminatory or that limits free movement of their respective citizens.
David Cameron has outlined plans to limit the migrant worker's access to benefits for four years as part of his proposals to renegotiate Britain's membership of the European Union.
"We can support those elements of UK reform proposals with a potential to modernise the European Union, especially as concerns increasing competitiveness and a stronger role of national parliaments," the four countries, called the Visegrad group, said.
"However, as the Visegrad Group countries consider the freedom of movement one of the fundamental values of the European Union, proposals regarding this area remain the most sensitive issue for us. In this respect, we will not support any solutions which would be discriminatory or limit free movement", the statement added.
Poland could drop fierce opposition on plans to cut EU benefits in return for a deal on new NATO bases, the country's foreign minister said.
Britain leaving the EU risks leading to the break-up of the UK, while "gravely weakening" the continent's security, William Hague warned.
The fact is if Britain leaves the EU a new relationship would have to be forged - we've no idea what that relationship would look like.