The true extent of organised crime in the UK is becoming clearer as a result of an extensive intelligence-gathering drive, according to the policing organisation responsible for bringing gangsters under control.

The Serious Organised Crime Agency (Soca) says it has identified 5,000 leading criminals – about 3,000 more than they knew about 12 months ago – and have a clearer picture of the impact that their crimes have upon society.

But Soca warned that the country needed to find innovative ways of dealing with organised crime, such as seizing assets, obtaining injunctions through the civil courts and enhancing protection for individuals and businesses at risk of being targeted by gangsters.

On the third anniversary of its launch, Soca said it had denied £175m to criminals in the UK over the last year through cash seizures and confiscation orders, and that a further £88m had been denied to criminals overseas. It said it had been involved in the seizure of more than 85 tonnes of cocaine, and issued 71 alerts intended to protect businesses from crime.

The agency is attempting to make more use of what it terms "lifetime management" techniques, including the imposition of serious crime prevention orders, which can limit criminals' travel, banking facilities or access to mobile telephones.

Soca has faced persistent criticism in recent months over the effectiveness of intelligence-gathering as opposed to more traditional policing methods.

Yesterday, the prime minster, Gordon Brown, used his first major speech about crime to reassure the agency that the government had no plans to close it down.

Sir Stephen Lander, the former head of MI5 who is chairman of Soca, said the agency would need up to two more years to prove its worth.

He and the agency's director general, Bill Hughes, a police officer, said their "compendium of tools" would be needed in addition to prosecutions if organised crime were to be effectively tackled.

"We now have a list of about 5,000 people where we have the intelligence and information that we require," said Hughes. "We have identified people who weren't on the radar before and we have also identified people who were on the radar but we didn't know exactly what it was they were doing. Now we have a better understanding of how they are operating and how best we can tackle them."