A group of Tory activists has urged David Cameron not to lift border controls on Bulgarian and Romanian migrants entering the UK on Wednesday.
In an open letter to the Prime Minister, Conservative Grassroots, a network of party members, called on Cameron to use a safeguard clause in EU law which would prevent a "wave of mass immigration".
They are the latest group to pledge their support to an amendment to the Immigration Bill, which would allow the UK to extend controls until 2018 because of "exceptional economic circumstances".
The Prime Mister has come under increasing pressure from his own party , following concerns that full access to the UK labour market would prompt damaging levels of immigration.
James Joshua, director of Conservative Grassroots, said: "In just a couple of days Britain faces a wave of mass immigration from Bulgaria and Romania at the end of the seven-year moratorium put in place by the last Labour government.
"Some estimates have suggested that more than 300,000 Romanians and Bulgarians will travel to the UK. This will put huge pressure on public services at a time when the country is struggling under a mountain of debt with on-going acute challenges within the economy."
The seven-year restrictions on Bulgaria and Romania were imposed when the two countries joined the EU in 2007. Under the limits, citizens could only obtain a visa if they were self-employed, had a job offer, or were given a specialist role.
Mr Joshua added that Conservative Grassroots "respectfully disagree" with the Government, which claims it does not lie within its power to extend the restrictions.
Robert Woollard, chairman of Conservative Grassroots, said: "I only hope that the Prime Minister stops running scared of this amendment and gives his own back benchers an opportunity to vote on it."
Backbench Tory MP Nigel Mills' proposed amendment to the Immigration Bill would prevent the two countries joining the European Union's freedom of movement rules by a further four years to 2018 and is supported by more than 50 Conservative MPs.
The Government has said it will be "business as usual" on 1 January at the UK border and it remains unclear if additional staff or measures are being put in place at airports and ferry ports in the event significant numbers arrive.
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