British security officials said Tuesday that an attempted cyber attack on the main opposition Labour Party during general election campaigning had failed.
The National Cyber Security centre (NCSC) - part of Britain's spy agency GCHQ - said it was "confident the party took the necessary steps to deal with the attack".
Labour described the attack which took place on Monday as "large-scale".
"The Labour Party followed the correct, agreed procedures and notified us swiftly," an NCSC spokeswoman said in a statement.
"The attack was not successful and the incident is now closed."
However, on Tuesday afternoon the main Labour party website was inaccessible.
Instead, the website showed a message from a cyber-security firm warning of a problem.
The party did not have an immediate comment.
It had earlier revealed that Monday's attempted hack had been repelled without an apparent data breach.
"We have experienced a sophisticated and large-scale cyber attack on Labour digital platforms," a party spokesman said.
"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," he added.
'Very serious'
Labour leader Jeremy Corbyn admitted he was worried about other potential attacks.
In a speech on the campaign trail in Blackpool, northwest England, Corbyn said the incident was "very serious", even if it had ultimately failed.
"But if this is a sign to come in this election, I feel very nervous about it all," he told supporters.
"Because a cyber attack against a political party in an election is suspicious and something one is very worried about."
Some campaign activities were slowed but they were restored early Tuesday and were now back to normal, according to the party.
Labour confirmed it had promptly reported the attack to the NCSC, which monitors and works to protect security systems.
The agency said it has been working closely with British political parties for several years "on how to protect and defend against cyber attacks", and had met the major parties last week ahead of the 12 December election.
There is lingering concern in Britain about the potential for outside interference in the general election, with major parties increasingly relying on digital messaging.
However, The Sun tabloid reported that GCHQ sources said there was no evidence to suggest any state-sponsored attack, and suggested the incident involved low-level hacking.
