Bulgarians and Romanians will gain the unrestricted right to live and work in the UK from December 2013, Home Secretary Theresa May has confirmed.
Temporary curbs imposed in 2005 to protect the British labour market are set to expire on that date.
The Labour Party have said they would support any moves to extend the ban.
But Mrs May told the BBC's Andrew Marr show that was not possible under EU law, although she was hoping to limit the impact on the UK economy.
The Home Office has not produced an official estimate of how many of the 29 million Romanian and Bulgarian citizens will take advantage of their new freedoms when controls are lifted.'Pull factors'
But some experts predict a large number of immigrants from Eastern Europe, which Labour has warned could put pressure on British jobs and wages.
The government's migration advisory committee says there is evidence Bulgarians would move to Britain because of its stronger economy, and it was "plausible" Romanians would come for the same reasons.
But extending restrictions could involve tearing up the provisions of the treaty signed when Romania and Bulgaria joined the EU.
Shadow Home Secretary Yvette Cooper last week told Andrew Marr that Labour would back an extension on the restrictions on migrants from the two countries working in the UK.
But Mrs May told the programme: "There are no further transitional controls we can put on. The transitional controls end in December 2013."'Sham marriages'
The government was looking instead at limiting access to benefits and the NHS to reduce the "pull factors" that encouraged migrants to come to the UK, she added.
She said she was working on possible limits to free movement with other EU nations - a "growing number" of whom were concerned about "sham marriages" and other abuses of the system.
The issue of freedom of movement will also be part of a review of the UK's relationship with the EU being carried out by Foreign Secretary William Hague.
The UK is also embroiled in a long-running legal dispute with the EU over the UK's habitual residence test, which limits benefit claims by new arrivals.
Mrs May insisted the government was still on target to cut annual net migration to the UK, from outside the EU, to below 100,000 by 2015.
The number of visas being issued had fallen following a clampdown on abuse of the system, she added, but said there was still a "huge amount of more work to do be done".