"If this is a sign of things to come in this election, I feel very nervous about it, because a cyber attack against a political party in an election is suspicious and something one's very worried about. A cyber attack, as happened in 2017, against our National Health Service is something that is incredibly dangerous to the health records and the health care and treatment of potentially millions of people."

Mr Corbyn said: "We do need far better defensive arrangements against cyber-attacks made against us. As to who undertook the attack, we're looking into all that at the moment and we have reported the attack to the National Cyber Security Centre, and they will be investigating it as well.

A Labour Party spokesperson described the attack on Labour's digital platforms as "sophisticated and large-scale", but 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."

The reported attack has occurred during a campaign for the 12 December general election in which all political parties are relying to a greater extent than ever before on digital methods to communicate with activists and get their message out.

It is understood that Labour activists complained on Monday of a slowdown in communications, suggesting that the attack targeted internal party networks.

Reports suggest it may have taken the form of a distributed denial of service (DDoS) attack, in which a network of compromised computers is used to flood the target with requests, overwhelming its server by creating an artificial spike in activity which slows down normal activities or even bringing them to a halt.

Although they can cause considerable disruption to targets, the consequence of a DDoS attack is generally short-term and normal activities are able to resume after a period of enforced downtime without a permanent loss of data or capacity.

Increased security measures have been put in place, and it is not thought that there was any loss of personal data belonging to supporters or activists.

That we experienced this kind of attack is of course a real concern, but I'm extremely proud of our incredibly professional staff team who took immediate action to ensure our systems and data are all safe #simplythebest https://t.co/yV3I8YvdM4 — Jennie Formby (@JennieGenSec) November 12, 2019

Labour's general secretary Jennie Formby said: "That we experienced this kind of attack is of course a real concern, but I'm extremely proud of our incredibly professional staff team who took immediate action to ensure our systems and data are all safe."

Party sources were remaining tight-lipped about the suspected source of the attack.

Labour's Stephen Doughty said that the incident made it all the more urgent that the Government should release the parliamentary Intelligence and Security Committee report on Russian attempts to interfere in the UK democratic system.

"This is yet another incredibly serious incident - and yet Johnson continues to supress the crucial report on Russian interference," said Mr Doughty, who is campaigning to retain his seat in Cardiff South & Penarth. "It must be published now. What are they hiding?"