Hackers have mounted a “sophisticated and large-scale cyber attack” on Labour’s digital platforms, the party has said.
A spokeswoman said the incident had been reported to the National Cyber Security Centre but they were confident that no data breach had occurred.
Some campaigning activities were temporarily slowed, but the systems were back up to speed by Tuesday morning.
“We have experienced a sophisticated and large-scale cyber attack on Labour digital platforms,” the 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.”
Labour sources later confirmed it was a so-called distributed denial-of-service (DDoS) attack – one of the most common forms of attack – although they would not be drawn on who they thought might be responsible.
In a DDoS, hackers flood a target’s online platforms with traffic from various sources, with the aim of slowing down access or causing websites to crash.
The last general election in 2017 was disrupted by the worldwide WannaCry attack, which hit hundreds of NHS trusts and GP practices, leading to thousands of appointments and operations being cancelled.
The NCSC subsequently attributed the attack to a shadowy North Korean crime organisation known as the Lazarus group.