Hackers claim they have attacked the National Crime Agency website in revenge for authorities arresting people for previous internet attacks.
The NCA website was down for more than two hours on Tuesday morning after being the subject of a Distributed Denial of Service attack, which involves overloading a website with too much traffic.
At about 9 a.m. the hacking group Lizard Squad tweeted a message reading: "Stressed out? http://www.nationalcrimeagency.gov.uk/ #offline."
It was accompanied by a picture of a lizard in a top hat and monocle - the Lizard Squad's Twitter picture with the National Crime Agency's official logo both superimposed over it.
Access to the website was restored just before 11 a.m., but it went offline very briefly again around half an hour later.
Last Friday, the NCA arrested six people as part of Operation Vivarium for allegedly purchasing a hacking attack tool which allows users to temporarily shut down websites.
Among the organizations believed to have been targeted are a national newspaper, a school, gaming companies and online retailers.
Those arrested are accused of buying code - known as Lizard Stresser - from Lizard Squad, which has claimed responsibility for the high-profile attacks on the Microsoft and Sony games networks last year.
The NCA said in a statement its site is an "attractive target" and "attacks on it are a fact of life". It added the hack was not a security breach and did not affect its "operational capability".
Such attacks are a "temporary inconvenience to users of our website," the NCA's statement went on to say.
The statement continued: "We have a duty to balance the value of keeping our website accessible with the cost of doing so, especially in the face of a threat which can scale up endlessly."
