The Labour Party says it was targeted in a "large-scale and sophisticated" cyber attack designed to take its systems "entirely offline".
A Labour spokeswoman said the cyber attack had "failed" because of the party's "robust security systems" and that they were confident that no data breach occurred.
The spokeswoman said they had reported the matter to the National Cyber Security Centre.
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."
The insider stated: "It was really very everyday, nothing more than what you would expect to see on a regular basis."
Mirror Online has contacted the National Cyber Security Centre for comment.
According to Sky News technology correspondent Rowland Manthorpe, with was a Distribute Deniable of Service (DDOS) attack.
He 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 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.
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.
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."
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