Serious organised crime is "increasing in scope and complexity", the agency set up to tackle it has said.
The Serious Organised Crime Agency (Soca) says that trafficking in Class A drugs, people smuggling and fraud are the biggest threats posed to the UK.
Its "threat assessment" is Soca's first report since launching four months ago.
The agency also says illegal immigrants are being smuggled into the UK from France for less than £150 and that criminals plan crimes in prison.
The report comes as the Home Office announced that the minimum amount of illicit cash that can be taken from a criminal is being cut from £5,000 to £1,000.
New crimes
The Soca threat assessment puts the cost of organised crime, including the costs of combating it, at £20bn.
READ THE REPORT
"The profits to be made from serious organised crime ensure that the various activities continue to be attractive, and the overall threat to the UK remains high, and is increasing in scope and complexity," director general William Hughes said in the report.
Developments in information technology had "both facilitated various crimes and spawned new forms of criminal activity", he added.
The report says that "time spent together in prison is the basis for many later criminal collaborations".
And it warns that criminals "at all levels" could easily get hold of guns.
"Working firearms, component parts and easily convertible blank-firing weapons can all be ordered via the internet and received through postal services," it adds.
Criminal lifestyles
Soca warns that the trafficking of Class A drugs "poses the single greatest threat to the UK".
The profits from drugs enabled more drugs to be bought and funded other forms of crime and supported criminal lifestyles, it said.
"It suggests that it is incredibly easy to get people into Britain clandestinely"
The report said that the costs to illegal immigrants to be smuggled into the country "varied substantially" from £150 - from France to the UK - up to £20,000 from China to the UK.
The suggestion that illegal immigrants were being smuggled into the UK from France for £150, is a much lower fee than has previously been suggested.
Sir Andrew Green, chairman of think-tank MigrationWatch, which campaigns against mass migration, said that represented "not much more than the usual fare".
"It suggests that it is incredibly easy to get people into Britain clandestinely."
But Keith Best, director of the independent Immigration Advisory Service, said he was "highly sceptical" about the figure of £150.
"Based on what has been said before, I would have thought the figure would be in the high hundreds, if not the thousands."
Corrupt relationships
The report also says that organised criminals use corruption to help their crimes.
Police officers and others involved in the criminal justice system had been targeted "for essentially defensive purposes" while money had been made through corrupt relationships with people working in the financial field.
"By seizing cash sums as low as £1,000, law enforcement agencies will be in a better position to target those petty criminals who think they are beyond the law"
"Similarly, corrupt insiders have been used to supply information about security measures at sites, vulnerabilities in systems, and the whereabouts and movements of high-value goods," the report added.
Meanwhile, the minimum amount of suspect cash that can be seized from criminals has been reduced from £5,000 to £1,000.
Home Office minister Vernon Coaker said: "We are determined to reduce the harm caused by serious organised crime and ensure that those involved in it are deprived of the financial benefits.
"By seizing cash sums as low as £1,000, law enforcement agencies will be in a better position to target those petty criminals who think they are beyond the law."
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