A group of senior Tory activists have joined calls for David Cameron to extend controls on Bulgarians and Romanians entering the UK.
Conservative Grassroots, a network of party members, are the latest to pledge support to an amendment that would prolong the restrictions rather than scrap them on New Year's Day.
The Prime Minister has come under increasing pressure from his own party to take action, following concerns that full access to the UK labour market would prompt a "wave of mass immigration" from Wednesday.
In an open letter they urge Mr Cameron to use a safeguard clause in EU law which they believe could extend controls until 2018 because of 'exceptional economic circumstances'.
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 UK imposed the seven-year restrictions on Romania and Bulgaria after they joined the EU in 2007 - only allowing citizens 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 says it is powerless 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' amendment to the Immigration Bill would extend transitional arrangements on the two countries joining the European Union's freedom of movement rules by a further four years to 2018.
Conservative support behind the amendment has been building and now stands at more than 50 MPs.
The Government has said it will be "business as usual" on January 1 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.Suggest a correction