All Bulgarian and Romanian citizens will be free to live and work in the UK after temporary controls in place since 2007 expire on Wednesday.
The UK has not released forecasts of migrant numbers but campaigners say up to 50,000 people a year could come.
Immigration minister Mark Harper said curbs on access to benefits would ensure those heading to the UK would contribute to the economy.
Romanian officials have said talk of an "invasion" is far-fetched.
Business leaders in London have said firms will benefit from having "access to a larger labour pool" and have warned that the debate about immigration has become "highly politicised".
Bulgarians and Romanians gained the right to visa-free travel to the UK in 2007, when their countries joined the EU.

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