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The Labour Party was targeted in a "large-scale and sophisticated" cyber attack designed to take its systems "entirely offline".

The "full functionality" of Labour's online campaign systems was "reduced" as a result, the party said.

Officials have reported the attack 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".

Labour Party leader Jeremy Corbyn said this afternoon that he is “very nervous” following the attack.

He told reporters: “It was a very serious attack against us, as far as we’re aware none of our information was downloaded.”

(Image: AFP via Getty Images)

And he added: “If this is a sign of things to come in this election, I feel very nervous about it all because a cyber attack in a general election is very suspicious.”

He said an attack against the NHS in 2017 had been “incredibly dangerous”.

Mr Corbyn said the party was “looking into” who was responsible for the attack.

A Labour spokeswoman said the attempt failed because of the party's "robust security systems" and officials are confident that no data breach occurred.

There is no indication yet about who was responsible.

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."

(Image: Getty)

A security official with knowledge of the attack told Reuters that it was not particularly sophisticated.

The insider stated: "It was really very everyday, nothing more than what you would expect to see on a regular basis."

A National Cyber Security Centre (NCSC) spokesman said in a statement: "The NCSC has worked closely with political parties for several years on how to protect and defend against cyber attacks.

"We met the major parties last week ahead of the General Election.

"In terms of this incident, the Labour Party followed the correct, agreed procedures and notified us swiftly.

"The NCSC is confident the party took the necessary steps to deal with the attack.

"The attack was not successful and the incident is now closed."

A Labour Party source confirmed it was a Distributed Denial of Service (DDOS) attack.

What is a DDOS attack? 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."

Sky News technology correspondent Rowland Manthorpe explained: "What that is is it involved bombarding websites or digital services with large numbers of traffic until they collapse.

"The important thing to know about this is this is not an extremely cunning, sophisticated kind of attack - it's crude, it's brute force, it's actually quite easy to do.

"This is, as far as cyber attacks go, this is the equivalent of a hammer, rather than a scalpel."

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.

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.

(Image: Getty Images)

These included exposing a campaign by the GRU, the Russian military intelligence service, of “indiscriminate and reckless cyber attacks targeting political institutions, businesses, media and sport”.

The vulnerability of Britain to cyber attacks was highlighted in May 2017 when the UK was among nations targeted in the WannaCry malware attack which clobbered hospitals, businesses and banks.

It led to 7,000 NHS appointments being cancelled.

A report into Russian interference in the US presidential election in 2016 found illegal interference from Moscow occurred in a "sweeping and systemic fashion", according to special counsel Robert Mueller.

(Image: Getty)

Former US presidential candidate Hillary Clinton has hit out at the government over the delay to releasing the report.

She said the delay was disgraceful given the upcoming election, referencing the proven Russian interference in the 2016 US election.

The former secretary of state said: "I find it inexplicable that your Government will not release a Government report about Russian influence. Inexplicable and shameful. You're having an election.

"People deserve to know what is in that report. We had a somewhat similar problem in 2016."