The Labour Party says there has been a “sophisticated and large-scale cyber-attack” on its digital platforms.
A Labour spokeswoman said the attack had “failed” because of the party’s “robust security systems” and they were confident that no data breach occurred.
It had been reported to the National Cyber Security Centre, she said.
Security procedures had “slowed down” some election campaign activities, she added, but they were now “back up to full speed”.
The party has not specified which of its platforms were targeted, but its main website, Facebook and Twitter accounts are functioning.
Voters are due to go to the polls on 12 December.
In a letter sent to Labour campaigners, Niall Sookoo, the party’s executive director of elections and campaigns, said: “Yesterday afternoon our security systems identified that, in a very short period 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 due to our robust security systems and the integrity of all our platforms and data was maintained.
“I would I like to pay tribute to all the teams at Labour HQ who identified this risk and acted quickly to protect us.”
The Labour Party spokeswoman said: “We have experienced a sophisticated and large-scale cyber-attack on Labour digital platforms.
“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.”