The notorious Russian hackers who claimed to have crashed the Buckingham Palace website yesterday have previously targeted JP Morgan and the Eurizosion song contest, it has been revealed, as well as issuing threats against the NHS.

KillNet, a group of so-called hacktivists loyal to Russian President Vladimir Putin, have sought to claim the credit for a “denial of service attack” which took the royal site offline for an hour and a half yesterday morning, from 10am onwards.

The attack came just days after King Charles condemned the invasion of Ukraine, ordered by Putin in February 2022.

In a Telegram post, Hacker KillMilk, the group’s supposed leader, claimed responsibility.

KillMilk regularly posts on the messaging service and cyber security experts think the organisation he appears to lead has close links with Putin’s regime.

KillNet has carried out attacks on government institutions and private companies across the world, having founded weeks after Putin’s invasion.

In 2022, the Five Eyes intelligence network, which consists of agencies from Canada, Australia, New Zealand, the US and the UK, warned KillNet was one of several hacker groups which had vowed to attack anyone who attacked Russia or supported Ukraine.

KillNet is known for carrying out both Denial of Service (DoS) and Distributed Denial of Service (DDos) attacks, which make servers and computers unavailable by disrupting the services of the host which connects them to the internet.

In the past the group has threatened to shut down ventilators in NHS hospitals in retaliation to the arrest of one of its members, attacked the annual song contrast to try and stop Ukraine winning, which it is did, and also target the giant US bank.

After the arrest of a 23-year-old alleged KillNet member in Tottenham last year, after Romanian government websites were attacked, KillNet warned, also on Telegram: ‘If he is not released within 48 hours I will destroy your Romania, Great Britain and Moldova.

“I will destroy your entire information structure and even your Ministry of Health. All ventilators will be attacked.

“Appropriate response. Only then will you begin to realise the mistake you have made.”

Hackers also tried to crash the livertream of the final of the Eurovision song contest in Turin last year and disrupted the public vote using an army of bots, The Times has reported – although the firewall held, ultimately.

KillNet has also threatened JP Morgan with a DOS attack although a statement issued by the bank said while it was “aware of claims made by a threat actor group” it had ‘not experienced operational impact from this situation.’

A denial of service attack it is not the same as hacking, in which whoever is responsible actually gets into a website and the site is compromised.

Neither the content nor the systems of the royal website were accessed on Sunday.

It is currently unknown who is responsible for the cyber attack, an insider said said.

The site is now up and running as usual.