Bulgarian and Romanians coming to work in the UK are preparing to face more restrictions than other EU citizens, as they join the union on 1 January.
Romanians and Bulgarians will face added work restrictions
The Home Office had earlier revealed its plans to limit the right to work.
Unskilled workers will initially be limited to quota-based schemes. The skilled can work if they get a permit, are a student, or are self-employed.
The Conservatives say allowing any EU citizen to work in the UK if they are self-employed is a "big loophole".
Speaking on Sunday, shadow home secretary David Davis said the measures were "open to abuse" and dismissed Labour talk of a managed flow of workers as "a piece of spin".
"This is why we called some months ago for proper restrictions on workers coming from these two countries," he said.
The Tories, as well as the Lib Dems, say they fear the restrictions will simply push more people into working illegally.
Under government measures, Bulgarians and Romanians could also come to the UK if they qualify under the Highly Skilled Migrant Programme.
Quota schemes for low-skilled workers fill vacancies in the agricultural and food processing sectors.
The new system will be reviewed within 12 months, said the Home Office.
The government's proposed new Migration Advisory Committee will assess the needs of the UK labour market, the position of other EU countries, and the impact made by the eight poorer countries which joined the EU in 2004.
On-the-spot fines of £1,000 and penalties for employers of up to £5,000 are in place to penalise law-breaking, said the government, and Customs officials will work on tackling bogus self-employment.
From the first day of 2007 the EU will have 27 members - up from 15 three years ago - and a population of half a billion.
Home Office Minister Liam Byrne said a bigger Europe was good for Britain and the economy.
"But we need to understand the transitional impacts from the last round of accession before we take the next step. The measures in place from today will allow us to manage the flow of workers from Bulgaria and Romania as part of this process.
"Through this measured response to accession we will ensure that migration is, and continues to be, managed in the best interests of the country as a whole."
But UK Independence Party (UKIP) leader Nigel Farage warned of a "massive migratory flow".
"The average salary in Romania is about £30 a week and they're now joining us in political union," he said.
"We're saying to them 'we have an open door and if you want to come and live and work in Britain you can'."
The Home Office has been funding a publicity campaign in Bulgaria and Romania to discourage workers travelling to Britain without a work permit.
Some 8,000 posters have been put up and 40,000 leaflets distributed across the two countries.
According to reports the campaign, being run by the International Organisation for Migration (IOM), will last until the end of March.
The new curbs contrast with the "open-door" policy adopted in 2004, when 15,000 migrant workers were expected to arrive each year. Instead 600,000 arrived in two years.
Home Secretary John Reid said the policy had been a success because migrant workers had filled skills gaps.
But he acknowledged some schools had had to cope with a "significant rise" in pupils, while some councils had reported overcrowding in private housing.