Ninety senior Conservative activists have joined calls for the Prime Minister to extend controls on Bulgarians and Romanians entering the UK.
In an open letter, Conservative Grassroots , a network of party members, wants David Cameron to a use clause in EU law because of "exceptional economic circumstances".
They are the latest to call for prolonging the restrictions rather than scrapping them as planned on New Year's Day.
The letter argues that full access to the UK labour market would prompt a "wave of mass immigration" and called for an "emergency" recall of Parliament.
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, like every other EU country, imposed the seven-year restrictions on Romania and Bulgaria after they joined in 2007 - only allowing citizens a visa if they were self-employed, had a job offer, or were given a specialist role.
The Prime Minister has said, in the past, that he is unable to act to extend the transitional controls.
Mr Cameron has been under substantial pressure in recent months to be tough on immigration ahead of European elections next year.
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.
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