David Cameron has been told by EU ministers in Eastern Europe that free movement rights of migrant workers is a "red line" and non-negotiable.
The warning comes as the newly re-elected Prime Minister, heading a majority Tory Government , prepares to renegotiate Britain's membership of the 28-nation bloc.
Following his win at the polls, Mr Cameron restated his commitment to hold an in/out referendum by 2017, based on a new settlement with Brussels.
Poland's Europe minister Rafal Trzaskowski told the Financial Times: "We are ready to sit at the table and talk about what needs to be reformed ... but when it comes to immigration, our red lines are well known.
"Poland's strategic interest is to keep Britain in. But it does not mean we will agree to anything. Competition and the internal market are sacrosanct. And so is freedom of movement."
Slovakia's Europe minister Peter Javorcik said: "They cannot be touched."
And Szabolcs Takacs, Hungary's EU minister said freedom of movement was a "red line".
He added: "We don't like it when Hungarian workers are called migrants, they are EU citizens with the freedom to work in other European countries."
However, leaders in Western Europe have indicated they would assist Mr Cameron in reaching a renegotiated deal.
And European Commission president Jean-Claude Juncker has told the PM: "I stand ready to work with you to strike a fair deal for the United Kingdom in the EU."
Eurosceptic former cabinet minister David Davis said the priority would be securing an effective opt-out for the UK.
He said: "Freedom of movement is important, but it is not the main one.
"The main one is that we are able to say in future to the Europeans that 'this is too far for us'. Not a veto but an opt-out."