Chinese spies are likely to use details stolen by hacking the elections watchdog to target dissidents and critics of Xi Jinping’s Government in the UK, British intelligence services believe.

The UK Government has publicly blamed Beijing for targeting the Electoral Commission and behind a campaign of online “reconnaissance” aimed at the email accounts of MPs and peers.

A front company, Wuhan Xiaoruizhi Science and Technology Company, and two individuals, Zhao Guangzong and Ni Gaobin, linked to the APT31 hacking group have been sanctioned in response to the malicious cyber activity against the parliamentarians.

Foreign Secretary Lord Cameron said the actions were “completely unacceptable” and he had raised the issue with his Chinese counterpart Wang Yi.

The Chinese ambassador has also been summoned to the Foreign Office to account for his country’s actions.

Deputy Prime Minister Oliver Dowden, who announced the measures in a Commons statement, said: “The UK will not tolerate malicious cyber activity targeting our democratic institutions.”

The Electoral Commission attack was identified in October 2022. Still, the hackers had accessed the commission’s systems containing the details of tens of millions of voters for over a year.

The registers held at the time of the cyber attack include the name and address of anyone in the UK registered to vote between 2014 and 2022 and those registered as overseas voters.

The National Cyber Security Centre (NCSC), part of GCHQ, said it was likely that Chinese state-affiliated hackers stole emails and data from the electoral register.

This, in combination with other data sources, was highly likely to have been used by Beijing’s intelligence services for large-scale espionage and transnational repression of perceived dissidents and critics based in the UK.

There is no suggestion the hack impacted the largely paper-based UK electoral system.

John Pullinger, chair of the Electoral Commission, said the announcement “demonstrates the international threats facing the UK’s democratic process and its institutions” but insisted the attack had “not had an impact on the security of UK elections”.

APT31 almost certainly carried out a separate campaign against MPs and peers in 2021, officials said, with most of those targeted being prominent critics of the Chinese Government.

Parliament’s security department identified and mitigated the cyber campaign before any accounts could be compromised, the NCSC said.

Former Tory leader Sir Iain Duncan Smith, a member of the Inter-parliamentary Alliance on China (Ipac), said critics of the Beijing government “have been subjected to harassment, impersonation and attempted hacking from China for some time”, but MPs would not be “bullied into silence by Beijing”.

He added: “We must now enter a new era of relations with China, dealing with the contemporary Chinese Communist Party as it is, not as we would wish it to be.

“Today’s announcement should mark a watershed moment where the UK takes a stand for values of human rights and the international rules-based system on which we all depend.”

Foreign Secretary Lord Cameron said: “It is entirely unacceptable that China state-affiliated organisations and individuals have targeted our democratic institutions and political processes.

“While these attempts to interfere with UK democracy have not been successful, we will remain vigilant and resilient to the threats we face.”

The UK acted with support from allies in the Five Eyes intelligence-sharing partnership, including the US, Canada, Australia and New Zealand, in identifying the Chinese-linked cyber campaigns.

Mr Dowden said: “I hope this statement helps to build wider awareness of how state-sponsored cyber operations are targeting politicians and those involved in our democratic processes around the world.”

The Chinese Government vehemently denied that it had carried out, supported or encouraged cyber attacks on the UK, describing the claims as “completely fabricated and malicious slanders”.

A spokesperson for China’s embassy in London said: “China has always firmly fought all forms of cyber-attacks according to law. China does not encourage, support or condone cyber attacks. At the same time, we oppose the politicisation of cyber security issues and the baseless denigration of other countries without factual evidence.

“We urge the relevant parties to stop spreading false information and stop their self-staged, anti-China political farce.”

With local elections in May and a general election later this year, the NCSC has updated advice for political organisations, including parties and think tanks, to reduce the risk of cyber attacks.

Home Secretary James Cleverly insisted the upcoming elections were “robust and secure”.

NCSC director of operations Paul Chichester said: “The malicious activities we have exposed today indicate a broader pattern of unacceptable behaviour from China state-affiliated actors against the UK and worldwide.

“The targeting of our democratic system is unacceptable, and the NCSC will continue to call out cyber actors who threaten the institutions and values that underpin our society.”

The APT31-linked front organisation and individuals will be hit with an asset freeze and travel ban under the sanctions regime.

The announcement increased Tory pressure on Rishi Sunak to take a tougher line on China, including labelling it as a threat to the UK.

On a visit to Barrow on Monday morning, the Prime Minister repeated the language used in the Government’s integrated review of defence and foreign policy: “We’ve been very clear that the situation now is that China is behaving in an increasingly assertive way abroad, authoritarian at home and it represents an epoch-defining challenge, and also the greatest state-based threat to our economic security.”