The UK faces the threat of increased crime when Romania and Bulgaria join the EU, ministers have been warned.
Romanians and Bulgarians will be free to live in the UK
According to a government dossier reportedly seen by the Sun, up to 85% of UK cash machine crime is already committed by Romanian-organised groups.
The memo raises fears "free movement" will attract more people to organised crime in the UK, the newspaper says.
The Home Office says it warned in September that "necessary safeguards" must be in place for the change.
The document, drawn up by ministers and seen by a Cabinet committee last week, reportedly warns of increases in cash machine crime, sex trafficking, street violence and fraud.
A Home Office spokesman said that, while it did not comment on leaked documents, tackling organised crime was a "top priority for the government".
"The establishment of the Serious Organised Crime Agency (Soca) earlier this year signals our determination to make the UK one of the toughest places in the world to operate," he added.
He pointed out that, in a speech to the Police Superintendents Association conference on 19 September, John Reid said UK law enforcement agencies had been working closely with the Romanian and Bulgarian governments to "identify new challenges".
In his speech, Mr Reid told senior officers the planned expansion of the EU would have an impact on policing and crime.
Mr Reid spoke in September about the policing implications of the change
"We need to consider very carefully all the implications of the accession of new states and ensure that we have all necessary safeguards in place right down to local borough level to reassure the public that this movement will be managed fairly and competently," Mr Reid said at the time.
The spokesman added that a Home Office-led task force made up of the police, Soca and other agencies was already working on "measures to identify and tackle the full range of law enforcement issues that may arise".
Last week, Mr Reid unveiled limits on Bulgarians' and Romanians' right to work in the UK, saying the only unskilled working allowed would be in food processing and agriculture under a specific scheme with 20,000 places.
The government has exercised its right under the EU rules to restrict access for these workers for up to seven years.
The countries join the EU in January and Mr Reid is keen to avoid the large influx seen from the eight ex-communist states which joined the EU in 2004.
EU rules mean Romanians and Bulgarians will be free to live in the UK and to take any job if self-employed.