A London schoolboy has been secretly arrested over the “world’s biggest cyber attack” as part of an international swoop against a suspected organised crime gang.

The 16-year-old was detained by detectives at his home in south-west London after “significant sums of money” were found to be “flowing through his bank account”. He was also logged on to what officials say were “various virtual systems and forums” and had his computers and mobiles seized as officers worked through the night to secure potential evidence.

The boy’s arrest, by detectives from the National Cyber Crime Unit, followed an international police operation against those suspected of carrying out a cyber attack so large that it slowed down the internet.

The “distributed denial of service” or “DDoS” attack was directed at the Dutch anti-spam group Spamhaus which patrols the web to stop prolific spammers filling inboxes with adverts for counterfeit Viagra, bogus weight-loss pills and other illegal products.

Details of the arrest, which happened in April, had been kept secret, but have been disclosed to the Evening Standard ahead of the formation of the Government’s new National Crime Agency.

It will take over the National Cyber Crime Unit as part of a drive against offending carried out over the internet, now seen as one of the most serious crime-fighting challenges.

More than half of the 4,000 officers who will form the new agency next month will be trained in combating cyber crime.

The arrest of the London schoolboy, whose identity has not been disclosed, came during a series of coordinated raids with international police forces. Others detained included a 35-year-old Dutchman living in Spain.

A briefing document seen by this newspaper on the British investigation, codenamed Operation Rashlike, states that the attack was the “largest DDoS attack ever seen” and that it had a “worldwide impact” on internet exchanges. The document says services affected included the London Internet Exchange and that although the impact was eventually “mitigated” it managed to cause “worldwide disruption of the functionality” of the internet.

Giving details of the schoolboy’s alleged involvement, the briefing note states: “The suspect was found with his computer systems open and logged on to various virtual systems and forums. The subject has a significant amount of money flowing through his bank account. Financial investigators are in the process of restraining monies.”

The boy has been released on bail until later this year.

The disclosure of his arrest follows two cyber attacks on banks. Four men have appeared in court over the first, involving an alleged plot to take over Santander computers by fitting a device during maintenance work.

Another eight were arrested over a £1.3 million theft by a gang who took control of a Barclays computer.

Meanwhile, security minister James Brokenshire said the creation National Crime Agency would bolster efforts to combat organised criminals operating on the internet and ensure that “cyber gangsters” were left with no hiding place.

“The new National Crime Agency’s Cyber Crime Unit will pursue the organised crime gangs behind the online crimes that blight people’s lives and cost the economy millions,” he added.