UK 'cannot extend' migrant controls, says Grant Shapps
The UK has done all it can "within the law" to delay the lifting of work restrictions on Bulgarian and Romanian migrants, a top Conservative has said.
Party chairman Grant Shapps said the deadline for ending temporary controls had already been extended by two years and this could not be done again.
Tory activists say there is scope within EU law to retain controls until 2018 under exceptional circumstances.
And the UK Independence Party has said the UK's position is "total madness".
Bulgarians and Romanians gained the right to visa-free travel to the UK in 2007, when their countries joined the EU.
But since then, they have been able to work in the UK only if they are self-employed, have a job offer, or are filling specialist posts for which no British worker can be found.
These temporary restrictions come to an end on Wednesday, which has prompted warnings from Conservative activists of a "hugely disruptive and destabilising wave of mass immigration".
Ninety party members have written to David Cameron pressing him to use a clause in EU law - allowing countries to continue with border controls if they have "serious labour market disturbances" - to extend restrictions until 2018.
This comes on top of more than 60 MPs signing an amendment to the current immigration bill going through Parliament calling for a further five-year extension.
Mr Shapps said he had sympathy with those worried about the economic and social impact of future migration from Bulgaria and Romania, on top of that seen since Poland and nine other countries joined the EU in 2004.
But he said the "maximum" seven-year limit for transitional controls on migrants after their countries join the EU could not be extended under current arrangements.
"We are doing everything we can within the current law," he told Radio 4's World at One programme. "You can only act within the law. that is the basic principle of democracy."
Only by renegotiating the UK's membership of the EU, he suggested, could new rules be put in place in future to limit freedom of movement among citizens of existing and new EU members.
David Cameron has said the idea of freedom of movement within the EU, one of its most important principles, needs to be reconsidered as "massive" population shifts in the past decade have put pressure on countries across Europe.
Mr Shapps said: "Of course, this is why we want an EU in-out referendum so we can negotiate these things so these transitional controls can last longer in future.
"So for example, if you've got countries joining the EU in future where the standard of living is so incredibly different, then you are able to have much longer transitional controls or you don't allow people to travel until their countries and economies have come up to a certain level."
The government is tightening the rules from 1 January to ensure that migrants cannot claim out-of-work benefits for three months after arriving and will only qualify for support after six months if they had a genuine chance of employment.
Overseas visitors and migrants are also to face new charges for some NHS services in England.
But in a letter, written by Conservative Grassroots chairman Robert Woollard, the activists say the government's position is "perplexing" and "politically untenable".
They warn that long-term youth unemployment is already among the highest in Europe and pressure on job markets from further migration could lead to "social unrest", with local authorities and public services unable to cope.
They say a "safeguard clause" written into the EU treaties "allows for the re-imposition of temporary restrictive measures in any member state if it is 'undergoing or foresees serious labour market disturbances'.
Extending the controls to 2018 would allow the UK economy the space and time to reverse the long-term high youth unemployment trend, they add.
Ministers have refused to predict how many migrants might arrive from Romania and Bulgaria.
With eight other EU countries - including France, Germany and Spain - lifting restrictions at the same time, they say such forecasts are difficult.
Romanian and Bulgarian officials have suggested they believe about 8,000 migrants will come every year for five years but campaigners for tighter laws have said the figure could be as high as 50,000.
And UKIP leader Nigel Farage told Radio 2's Jeremy Vine show that it was "total madness in two days' time to open up our borders to hundreds of thousands of new people from Bulgaria and Romania".
"I don't think we need or want any more immigration, by that I mean settlement, in Britain until we sort out the current mess," he said.