By Frank Gardner
BBC security correspondent
Britain's vital infrastructure is at serious and mounting risk of suffering a concerted attack by computer hackers, according to government officials tasked with internet security.
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The BBC has learned that Britain, along with other western countries, has been under daily "cyber attack" from foreign intelligence agencies trying to steal secrets through the internet.
But none of these compares with the unprecedented cyber attack launched earlier this summer on Estonia by computers traced to Russia.
Estonia is now calling for an international convention on "cyber crime", while Nato is considering how best to respond to this 21st Century threat.
Estonian officials say they were subject to a three-week cyber attack from Russia - sparked by the relocating of a Soviet war memorial in the capital Tallinn.
Websites in Russia posted instructions on how to bombard Estonia using what is known as "botnets" - or "robotic networks", the officials say.
Such networks involve a group of compromised computers known as "zombies" and they can be anywhere in the world.
Without their owners knowing, distant hackers can instruct them over the internet to send out millions of bogus messages, overwhelming the bandwidth of the website and causing it to freeze or crash.
The result in Estonia was that banks, businesses, ministries, newspapers and political parties were temporarily incapable of conducting any of their business online.
Estonia's drastic response was to deny access to any incoming messages from outside the country - meaning that Estonians abroad, for example, were unable to access their bank accounts.
"We saw several massive focused and co-ordinated attacks," says Estonia's defence minister Jaak Aviksoo.
"In addition to individual people attacking, large botnets, including tens of thousands of computers distributed worldwide in a number of countries from China to Brazil to US, to South Africa and last but not least Russia and European countries," he said.
The Kremlin denied any involvement.
Could this happen in other countries, like Britain?
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The short answer, says Paul Docherty of Portcullis Security, is yes.
The more wired to the internet Britain becomes, the more vulnerable it is to cyber attack.
Mr Docherty said: "There is a high likelihood that a targeted attack in a similar manner would be very successful in the UK.
"It could potentially damage financial systems, it could potentially damage many other systems that are interconnected."
To help protect British infrastructure, the Centre for Protection of National Infrastructure has teams working continually to prevent hackers from attacking the system.
They also run a Computer Emergency Response Team to cope with any emergencies.
Their officials believe that the risks of a concerted cyber attack on the UK are likely to increase over the next three to four years - partly owing to the convergence of global telecoms which are switching over to internet-based protocols.
But already, they say, foreign intelligence agencies have been quietly trying to access secure information over the internet on defence, aeronautics, genetics and IT.
It is, they say, a constant unseen battle.
So what would a concerted cyber attack on Britain mean in practice?
It does, of course, depend on the target, but Mr Docherty uses one event in Australia as an example.
"An individual connected to the water system pumped raw sewage out into a field, into the back of hotels, into a river, affecting the drinking supply and affecting the climate around the hotel. All of this was done via his laptop computer.
"He knew he had access to a system, he knew a way of just automating the pumping of sewage.
"So he pumped millions of gallons which manually took months and months and months to repair, whereas it took him a minimal amount of time to affect the system."
What happened in Estonia has certainly been a wake-up call for internet security.
As technology in Britain and other countries races ahead, our vulnerability to cyber attack increases day by day.
The next time a country is attacked in this way it is likely to be more sophisticated and potentially even more damaging.