UK law enforcement and businesses are losing the "cyber arms race" with online attackers, the National Crime Agency (NCA) has warned.
Criminals' technical capabilities develop so fast they "outpace" Britain's response to the threat from hackers and cyber attackers, a new NCA report has found.
A key threat to the UK comes from a few hundred criminals in international gangs, some of which are such well-developed networks that they run call centres and employ translators.
A report detailing the NCA's annual assessment of cybercrime, put together with an industry group, said: "The accelerating pace of technology and criminal cyber capability development currently outpaces the UK's collective response to cybercrime.
"This 'cyber arms race' is likely to be an enduring challenge, and an effective response requires collaborative action from government, law enforcement, industry regulators and, critically, business leaders."
The Government is set to spend £1.9 billion over the next five years on defending the country against cyber attacks, and the NCA said law enforcement had also boosted its efforts, but businesses needed to do more.
Cyber attacks on firms appear to be massively under-reported. Official figures suggest there were 2.46 million "cyber incidents" in 2015, of which around 716,000 were reported to Action Fraud.
In May, director of the National Crime Agency's computer crime squad Jamie Saunders called under-reporting "a serious problem", while the national lead for economic crime Commander Chris Greany said police needed more information to build "a clearer picture of the threat".
The report said: "Directors also have an important role in addressing the under-reporting of cyber crime which continues to obscure the full understanding of, and hence responses to, cyber crime in the UK.
"In particular, we urge businesses to report when they are victims of cyber crime and to share more intelligence, both with law enforcement and with each other."
Cyber attacks cost the UK billions of pounds per year and the bill is growing, the NCA believes. The most serious threat comes from international gangs.
"Cyber criminals targeting the UK include international serious organised crime groups as well as smaller-scale, mostly domestic, criminals and hacktivists (hacker activists)," it said.
"The NCA assesses that the most advanced and serious cyber crime threat to the UK is the direct or indirect result of activity by a few hundred international cyber criminals, typically operating in organised groups, who target UK businesses to commit highly profitable malware-facilitated fraud.
"These cyber attacks include attacks directly targeting business systems and attacks against individuals."
Plans to improve the UK's cyber defences include a new National Cyber Security Centre and to undertake work with internet service companies to block attacks.