Jeremy Corbyn has said the cyber attack against the Labour Party was “very serious” and “worrying” – especially during an election.
Labour said it was targeted in a “large-scale and sophisticated” cyber attack designed to take its systems “entirely offline”.
But insist their security measures were up to the job.
Mr Corbyn said: “We have a system in place in our office to protect us against these cyber attacks, but it was a very serious attack against us.
“So far as we’re aware none of our information was downloaded and the attack was actually repulsed because we have an effective in-house developed system by people within our party.”
But speaking in Blackpool the Labour leader expressed concern about possible future attacks.
He said: “If this is a sign of things to come in this election, I feel very nervous about it all because a cyber attack against a political party in an election is suspicious, something one is very worried about.
“A cyber attack, as happened in 2017 against our NHS, is something that is incredibly dangerous to the health records and the healthcare and treatment of potentially millions of people and so we do need far better defensive arrangements against cyber attacks made against us.”
Officials have reported the attack to the National Cyber Security Centre.
But there is no indication yet about who was responsible.
Mr Corbyn added: “As to who undertook the attack, we’re looking into all that at the moment and we’ve obviously reported the attack to the National Cyber Security Centre and they will be investigating it as well.”
Niall Sookoo, the party’s executive director for elections and campaigns, said in a letter to colleagues: “Yesterday afternoon our security systems identified that, in a very short space of time, there were large-scale and sophisticated attacks on Labour Party platforms which had the intention of taking our systems entirely offline.
“Every single one of these attempts failed because of our robust security systems and the integrity of all our data platforms was maintained.”
He said the attacks had, however, reduced “full functionality” of campaign systems.
“We have experienced a sophisticated and large scale cyber attack on Labour digital platforms,” a party spokeswoman said.
“We took swift action and these attempts failed due to our robust security systems. The integrity of all our platforms was maintained and we are confident that no data breach occurred.
“Our security procedures have slowed down some of our campaign activities, but these were restored this morning and we are back up to full speed.
“We have reported the matter to the National Cyber Security Centre.”
A security expert said this kind of attack involved “bombarding websites or digital services with large numbers of traffic until they collapse”.
A National Cyber Security Centre (NCSC) source has said the cyber attack on the Labour Party was low level and that there was no evidence of sponsored activity.
DDoS attacks direct huge amounts of internet traffic at a target in an effort to overwhelm computer servers, causing their software to crash.
They are often carried out via a network of hijacked computers and other internet-connected devices known as a botnet.
It comes as Tories face increasing pressure to release a report examining Russian influence in British politics.
Although there has been no indication of the precise contents of the report, it will assess the threat posed by Moscow to Britain’s democratic processes following an 18-month inquiry into illicit Russian activities in Britain.
A number of Labour MPs have suggested the attack on Labour could have been a state actor.
A Labour source says the party was targeted in a distributed denial-of-service (DDOS) attack.
This happens when hackers attempt to overwhelm a website with more traffic than its servers can handle.
Netflix, Spotify and Amazon are among the firms that have been targeted in the past.
According to online security firm Norton, it is “one of the most powerful weapons” on the internet.
The firm says: “A distributed denial-of-service (DDoS) attack is one of the most powerful weapons on the internet. When you hear about a website being ‘brought down by hackers,’ it generally means it has become a victim of a DDoS attack.
“In short, this means that hackers have attempted to make a website or computer unavailable by flooding or crashing the website with too much traffic.”
Stephen Doughty, who sits on the Home Affairs Committee, said: “This yet another incredibly serious incident – and yet Johnson continues to supress the crucial report on #Russia interference. It must be published NOW. What are they hiding?”
Clive Lewis tweeted: “My, my, dirty tricks have a come along way from tearing down Labour garden boards & extracting leaflets from letterboxes. Yet another reason the withheld ‘Russian Report’ should be released, immediately.”
In the build up to the general election on December 12, parties will be reliant on digital methods of reaching voters on an unprecedented scale.
Last month the National Cyber Security Centre revealed that more than 650 attacks had been made against the UK in a year.