Hacker group LulzSec claims that it has attacked the website of the Serious Organised Crime Agency (Soca).
The group recently carried out a distributed denial-of-service (DDoS) on the CIA, and has now turned its attention to Soca.
The agency is currently helping Virgin Media in its efforts to protect its broadband customers against the Spyeye trojan.
The group announced the hacker attack on Monday afternoon on Twitter. "Tango down – www.soca.gov.uk – in the name of #AntiSec," it tweeted.
Reports confirmed that the Soca website was temporarily down but was back up and running in a matter of minutes.
LulzSec also said it had teamed up with another hacker group, Anonymous, in open cyber warfare against government agencies and big companies.
They are calling the collaboration "Operation Anti-Security".
"We encourage any vessel, large or small, to open fire on any government or agency that crosses their path. We fully endorse the flaunting of the word 'AntiSec' on any government web site defacement or physical graffiti art," LulzSec said in a statement.
"To increase efforts, we are now teaming up with the Anonymous collective and all affiliated battleships."