Work rights in the UK for Romanians and Bulgarians are likely to be restricted if they join the EU next year, Home Secretary John Reid has said.
Migrant workers are filling labour shortages, ministers say
He told the Police Superintendents Association in Chester the implications needed to be considered "carefully" to ensure "all safeguards are in place".
He emphasised the social dangers - including the risks posed by criminals - if there was unrestricted access.
An estimated 600,000 East Europeans have moved to the UK since 2004.
The majority of those are from Poland, which was one of eight former communist countries which joined the EU two and half years ago.
'Transitional controls needed'
In his speech, Mr Reid told senior officers the planned expansion of the EU would have an impact on policing and crime.
"That is why we need to manage immigration carefully, including in respect of the forthcoming decision over Romania and Bulgaria," he said.
"We need to consider very carefully all of the implications of the accession of new states and ensure that we have all the necessary safeguards in place ... to reassure the public that this movement will be managed fairly and competently."
He added: "I know that law enforcement agencies have already been working closely with the Romanian and Bulgarian governments to identify the new challenges."
Speaking in Leeds, Conservative leader David Cameron said he could see a case for "transitional controls", adding: "It's good to see Mr Reid has caught up with Conservative thinking."
The think-tank Migrationwatch, which campaigns against mass immigration, predicts that there could be 300,000 arrivals from Romania and Bulgaria over 20 months unless access to the labour market is restricted
Its chairman Sir Andrew Green said Mr Reid's comments marked "an important victory for those of us who believe that immigration must be brought under control".
"However, Romanians and Bulgarians will be free to come here as visitors so it will be difficult to enforce any restrictions on their working here unless the government at last gets tough on employers of illegal labour."
Ex-Cabinet minister John Denham has already urged the government to delay granting Bulgarians and Romanians free access to work in the UK, claiming that the country needed "breathing space" to absorb migrants from the earlier wave.
The UK was one of only three existing EU members to allow unrestricted work rights to the new member states.
There have been reports that, with pressure to bring in curbs given the much larger than expected influx since 2004, a work-permit system is being considered.
Ministers say the migrants have filled gaps in the labour market and helped keep inflation down.
The Guardian reported earlier this summer that relatively few potential migrants were expected to be able to show they have the skills needed to get the new work permits.
The permits would be similar to the points-based scheme used for migrants from outside the EU, which stresses university education, professional qualifications and potential earning power.
EU leaders are due to decide within days on whether to give the go ahead for Bulgaria and Romania to join the union at the start of next year, or whether to postpone their joining date.
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