While Mr Cameron’s election victory has been greeted positively from across Europe, Britain’s traditional allies in the east are already preparing for a fight to defend the free movement rights of migrant workers.

 “They cannot be touched,” Peter Javorčík, Slovakia’s Europe minister, told the Financial Times.

Szabolcs Takács, Hungary’s EU minister, called freedom of movement a “red line”, adding that it was one of the EU’s biggest achievements. “We don’t like it when Hungarian workers are called migrants, they are EU citizens with the freedom to work in other European countries,” he said.

Meanwhile Rafał Trzaskowski, Poland’s Europe minister, said: “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.”

Britain has in the past counted former communist countries in central and eastern Europe as natural allies, but Mr Cameron has hurt relations in recent years by his tough stance on migration.

José Manuel Barroso, former European Commission president, said on Monday the principle of free movement of people was inviolable but that Mr Cameron “had a point” in wanting to stop “some abuses of our social security systems” by migrant workers.

Mr Barroso said if Mr Cameron approached his renegotiation in a positive spirit it was possible to have “a good conversation” with other EU leaders. “The tone is very important,” he said.

He also told the BBC’s Today programme that the prime minister had “renewed legitimacy and greater internal authority” after this election victory and that Ukip had become “almost irrelevant” in the British debate.

“He is someone who is determined and pragmatic,” Mr Barroso said.

The Europe issue is set to become the biggest flashpoint in Mr Cameron’s pursuit of a “new deal” for Britain, which he will put to an in-out referendum on UK membership of the bloc by 2017.

His election victory last week was accompanied by signs from leaders in western Europe that they would try to help him reset Britain’s relationship with the EU.

Angela Merkel, German chancellor, described his win as “simply great” and François Hollande, French president, called Mr Cameron on the day of his victory to invite him to Paris. Jean-Claude Juncker, European Commission president, said: “I stand ready to work with you to strike a fair deal for the United Kingdom in the EU.”

Mr Cameron needs to secure a good deal, amid warnings by David Davis, a senior Conservative MP, that 60 or so Tory MPs could vote for a Brexit unless he succeeds.

Mr Davis wants Britain to be able to “opt out” of any EU rules it does not like — a demand unlikely to be agreed by Mr Cameron’s negotiating partners.

And Graham Brady, chairman of the Tory backbench 1922 committee, on Sunday night urged Mr Cameron to give all MPs, including ministers, a free vote in the EU referendum.

Mr Trzaskowski said Poland, like other countries, was willing to help but there were limits. “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,” he said.

Referring to Mr Cameron’s 12-seat Commons majority, Mr Trzaskowski said: “He is strong, he now does not have to pander to anti-European feelings . . . [or] be held hostage by a tiny minority of his party with anti-EU views.”

In his election manifesto, Mr Cameron promised a four-year waiting period for migrant workers claiming UK benefits, a measure British officials think will need EU treaty change and unanimous support from all 28 member states. Eastern European officials oppose anything establishing two tiers of worker rights that in effect discriminate against their citizens.

Some senior EU diplomats who will be closely involved in any renegotiation fear that Mr Cameron may overplay his hand on a toxic and emotive issue for voters in eastern European.

While there are past examples of Denmark and Ireland securing special deals on EU membership terms, none of these required other member states to make serious domestic political sacrifices.

Source: The Financial Times