German Chancellor Angela Merkel has dealt another blow to David Cameron saying she will not ‘tamper’ with EU freedom of movement.

The Prime Minister is relying on being able to make changes to half a billion people having the right to live, work and settle in the UK as he tries to stem the flow of support from the Conservative Party to UKIP.

The news, coming only days after it was revealed Mrs Merkel told Cameron to pay the extra bill of £1.7bn to Brussels will not be welcomed by Downing Street.

And with only weeks to go until the Rochester and Strood by election, where polls put UKIP’s Mark Reckless in the lead.

In an interview with The Sunday Times, Mrs Merkel said “Germany will not tamper with the fundamental principles of free movement in the EU.”

“I spoke to David Cameron and we agreed to assess the verdict together. These are controversial issues that are debated also in our country.

“I am of the opinion that they need to be resolved in a way that tackles abuse.”

A Romanian migrant took the German government to court to claim unemployment benefits despite not having had a job in the country.

“On the other hand we must not interfere with the fundamental principles of free movement in Europe.” she said, in a direct response to the Prime Minister’s claims he will put an end to unlimited immigration from the EU.

Mr Cameron promised to cut net migration to the tens of thousands, but the figure has topped 200,000 after a surge in the numbers using freedom of movement laws to move from elsewhere in the EU.

The Prime Minister has promised to renegotiate Britain’s membership of the EU, before holding an in-out referendum by 2017.

The concept of time limited national insurance has been mooted by senior Conservative figures but any rules which are applied to people from within the EU have to be applied to British people in order to meet EU rules.

Defence Secretary Michael Fallon denied that Mr Cameron’s efforts to renegotiate the UK’s relationship within the EU were also foundering.

“The negotiation itself hasn’t started yet,” he told the BBC’s Andrew Marr show. “We’ve set out the outlines, giving national parliaments more say over European legislation – who would disagree with that?”

“Stopping people moving around Europe to claim benefits, that is extremely important.”

“Having an open trading Europe that has less bureaucracy makes it easier to create jobs and wealth as we’ve done in this country.”

It has been a tough week for Mr Cameron as MEPs were in Strasbourg voting in the new European Commission and overturned all the cuts to the EU budget which national leaders proposed.

The new President Jean Claude-Juncker told one of his new commissioners to “promote freedom of movement” within the EU.

The latest poll by Ipsos Mori showed that the majority of Britons wanted only an economic relationship with the EU or total withdrawal. Since an economic relationship is not possible as a member of the European Union, this puts 51 percent of people in favour of UKIP’s EU policy.

Outgoing European Commission chief Jose Manuel Barroso insisted last week that an arbitrary cap on EU immigration would be incompatible with EU law.