Government-supported Get Safe Online has apologised for the collapse of its website after receiving a high volume of visitors.
The site had recently published information on how to protect computers against the GameOver Zeus Trojan.
Despite extra capacity being added, problems continued well into Wednesday morning. "This has been a learning curve for us," said Tony Neate, Get Safe Online's chief executive.
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He dismissed speculation of a denial of service attack, and said Get Safe Online is looking at why the site failed to find ways of ensuring that the site will be able to cope with high demand in future.
By Wednesday morning the site had stabilised after quadrupling its capacity.
Independent security advisor Graham Cluley has criticised the support site’s handling of the GameOver Zeus information campaign.
"If the government is going to rely upon Get Safe Online to distribute internet security advice, the site needs to invest in the infrastructure required to remain accessible at all times, every day of the year, because you never know when the next big security issue will crop up - and when the public will need advice urgently,” he told the BBC.
The announcement, warning of the Gameover Zeus threat, was made by the UK's National Crime Agency (NCA) just before 1400 BST on Monday. The NCA’s National Cyber Crime Unit has indicated that collaboration with private sector companies is one of its key strategies to tapping into the skills and resources vital to international cyber crime fighting.
The NCA said Get Safe Online's down time was "disappointing", but noted that the safety information is also available on the website of the UK's newly established Computer Emergency Response Team CERT-UK.
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The US government, Microsoft,Symantec and Trend Micro have also posted advice on how to protect business and personal computers, including links to malware removal tools.
Businesses have been advised to test their incident responses and business continuity plans and work with their IT departments to educate employees on the potential threat.
Russian Evgeniy Bogachev is suspected of being the ringleader behind the botnet distributing data-stealing Trojan GameOver Zeus, also known as GOZeus or P2PZeus.
The US has charged Bogachev with conspiracy, wire, bank and computer fraud, and money laundering.