By Mark Easton
BBC home editor
The agency is also targeting importers of cutting agents
The international cocaine market is "in retreat" after a year of successful operations around the world, the Serious Organised Crime Agency claims.
It says its undercover work has helped send wholesale prices soaring.
Prices per kilo have risen from £39,000 in 2008 to over £45,000 (50,000 euros), but street prices have remained stable.
However, new figures obtained by the BBC suggest almost a third of police seizures are now less than 9% pure, the lowest recorded purity level.
The data, collected by the Forensic Science Service, reveals how drug gangs are using increasing amounts of chemicals - so-called cutting agents - to dilute cocaine powder sold on the streets of Britain.
They include the cancer-causing drug phenacetin, cockroach insecticide and pet worming powder.
Soca head of enforcement Trevor Pearce told the BBC: "There is a discernible effect that we are now seeing in relation to the availability of cocaine both in Europe and also across the UK.
"We are now seeing high-quality cocaine at about £45,000 (50,000 euros) per kilo wholesale in the UK. That's significantly higher than it has been and has to be indicative of the pressure which the importers are under."
Soca has released its internal estimates of wholesale cocaine prices. Two years ago, the highest quality blocks typically cost £35,000 (38,946 euros). By the end of last year, the price had risen to £39,000 (43,393 euros).
But in the first few months of this year the price, according to Soca, has hit a record level; more than £45,000 per kilo.
Soca accepts that the rise in the price of wholesale cocaine measured in Sterling may in part be a consequence of the weakness of the pound.
But it argues that a similar price rise "across Europe" suggest the increases can be attributed to more than fluctuations in the money markets.
"We are seeing wholesale prices of cocaine rising across Europe, in Spain and Belgium. We think that is due to a large degree to the strategy of working in South America, the Caribbean, across the Atlantic and with European partners - tackling it in a different way," Mr Pearce said.
Soca - which has been under pressure to prove its effectiveness - claims its activities in the run-up to Christmas stopped 10 tonnes of street-quality cocaine powder being sold in Britain.
But analysts at Drugscope say the shortage of supply has not seen a fall in street prices although purity levels have dropped.
"At the moment price is relatively stable for cocaine," says Drugscope director Martin Barnes.
"What is happening is that dealers are maximising their profits by selling a product that is potentially more harmful and much less pure and a lot of people buying it probably don't realise that's what's going on."
The latest figures from the Forensic Science Service reveal how average purity of police seizures of cocaine has fallen to its lowest ever recorded level - less than 20% pure on average. Almost a third (31%) of seizures are below 9% pure.
Joy Newman, a scientist with the FSS, says there has been a significant decrease in the quality of the wraps of cocaine powder examined at the laboratory.
"Purities now are quite low - between 5% and 40% in police seizures and we have seen them as low as 4% or 5% in the last few weeks."
Since last October, every seizure of cocaine is tested for impurities as part of a new Soca initiative codenamed Project Endorse.
A searchable database has been constructed which has identified links between more than 170 seizures that would otherwise not have been connected.
The agency is also targeting importers of chemicals used as cutting agents, and has sent warning letters to some suppliers who they suspect may be selling the products to drug gangs.
Soca may have decided that, whatever you do on the war on drugs, you cannot win the vital public relations battle by operating from the shadows
In the last year there have been 72 "law enforcement operations" involving cutting agents, even though the chemicals themselves are legal.
Critics of Soca suggest trying to limit the supplies of illegal drugs is an unsustainable tactic.
Martin Barnes from Drugscope argues that while something very significant is happening to the cocaine market, "enforcement methods have a limited impact in terms of the availability of drugs and the key is to tackle the demand side".
Trevor Pearce from Soca accepts that the drug gangs are likely to change their tactics.
"One of the things we have to do is to try to anticipate where the organised criminals are going to next, so we can stay a step ahead of them," he said.