Hackers have attacked Britain's opposition Labour Party for the second time in two days, sources say.
The attackers flooded Labour's web services with malicious traffic in an attempt to force them offline just weeks ahead of a national election.
The party said earlier on Tuesday it had experienced a "sophisticated and large-scale cyberattack on Labour digital platforms", but that the attack was successfully repelled and no data was compromised.
Just hours later, the party's website and other online services came under a second digital bombardment, according to two people with knowledge of the matter, and documents seen by Reuters.
One of the sources said it was unclear if it was the same hackers or a copycat attack but there was currently nothing to link either incident to a foreign state.
A Labour spokesman had no immediate comment on the second attack, which the sources said was ongoing.
Britain's security agencies have warned that Russia and other countries could use cyber attacks or political messages on social media to attempt to disrupt the December 12 election.
Moscow has repeatedly denied Western allegations of election interference.
Britain's National Cyber Security Centre said the first attack was a distributed denial-of-service (DDoS) attack - a technique used by hackers to take down websites by overwhelming them with traffic.
"DDoS attacks are a common form of attack used by a very wide range of attackers. Mitigation techniques are available and worked in this case," a NCSC spokesman said.
The nature of such attacks often made it difficult to attribute responsibility to any particular group, he said.
Labour leader Jeremy Corbyn said the first attack was very serious but was successfully repelled by the party's defence systems when the digital assault began on Monday.
"But if this is a sign of things to come in this election, I feel very nervous about it all," he said.
Australian Associated Press