MI5 secretly warned the Conservative Party that two of its potential candidates to become MPs could be spies for the Chinese state, it is being reported.

The security service contacted the party about two people in 2021 and last year and advised that they should not be on the central list of candidates, according to a report in The Times today.

MI5 is said to have raised concerns that the pair had links to the United Front Work Department, a body charged with influencing global policy and opinion. They were blocked from the list, which is used as a pool to pick candidates for by-elections and general elections.

“It was made very clear that they posed a risk,” a source said. “They were subsequently blocked from the candidates list. They weren’t told why.”

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A Conservative Party spokesman said: “When we receive credible information regarding security concerns over potential candidates we act upon them.”

Details of the alleged attempt to infiltrate the party emerged after a Tory parliamentary researcher was arrested on suspicion of spying.

The man, who is in his twenties and from Edinburgh, was the director of an influential group on Beijing co-founded by the security minister. He was also employed as a researcher by Alicia Kearns, chairwoman of the Commons foreign affairs select committee.

The Scot released a statement through lawyers on Monday, insisting that he was “completely innocent”. He said the allegations were “against everything I stand for”, adding that he had spent his career “trying to educate others about the challenge and threats presented by the Chinese Communist Party”. China called the claims malicious slander.

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Ken McCallum, the head of MI5, has warned that the Communist Party poses “the most game-changing strategic challenge” to the UK. He said in July last year: “One of the things that is very striking is that they are prepared to invest in cultivating people at local level, potentially, and at the outset of their political career.”

In July, the Commons intelligence and security committee published a report saying that China was targeting the UK “prolifically and aggressively”, but that government departments did not have the “resources, expertise or knowledge” to tackle the threat.

Last year the security service took the unprecedented step of issuing an alert to MPs naming Christine Lee, an Anglo-Chinese lawyer, as an agent of influence carrying out “political interference activities on behalf of the Chinese Communist Party”. Ms Lee, 59, who denies wrongdoing and is suing MI5, had donated almost £500,000 to Barry Gardiner, Labour’s former shadow international trade secretary. Ms Lee’s son worked in Mr Gardiner’s office.

Tory candidates are vetted by the party. They are subject to criminal record checks and an “in-depth” due diligence report, and are interviewed to assess their political judgment, experience and integrity. A Tory party source confirmed to the Times that in the past the security service had given a “nudge” if it had concerns about candidates.

Part of MI5’s mandate is to offer physical and personnel protective security to businesses and other organisations, including those involved in politics. It is an advisory role and it cannot make decisions about the employment of individuals or their involvement in the political arena.

READ MORE: China spy row: Scot says he is 'completely innocent'

Prime Minister Rishi Sunak is under pressure from Tory MPs who have been sanctioned by Beijing to formally declare China a threat to the UK. The government has described China as an “epoch defining challenge”, but backbenchers want it to designate the country a hostile state.

The United Front Work Department (UFWD) is one of the most important departments of the Chinese Communist Party, accused of deploying shadowy tactics to exert the influence of Beijing both at home and overseas.

Earlier this year a report by parliament’s intelligence and security committee (ISC), which oversees the work of MI5 and other spy agencies, said that some of the UFWD’s work in trying to influence politicians and public perceptions of China might be seen as akin to traditional diplomacy.

However, it also had a remit to engage in domestic and international activity “with the purpose of ensuring that potential critics and threats to the CCP are influenced, co-opted or coerced into silence”.

The report warned: “UFWD’s remit includes engaging in political influence and interference operations overseas, to ensure that politicians and high-profile figures in foreign states are supportive of the CCP, or at the very least do not criticise China or counter its narrative.”

Many of its influence activities are directed at the Chinese diaspora.

Earlier this year a Times investigation into secret police stations, designed to intimidate Chinese who moved to the UK to seek freedom, prompted calls for a full inquiry into the work of the UFWD in this country.

Their official purpose is to help Chinese citizens overseas with administrative issues, such as renewing driving licences. However, there are also reports of the stations being involved in “persuade to return” operations.

An alleged secret police station operated by China in a Glasgow restaurant was closed earlier this year.

The Loon Fung restaurant, on Sauchiehall Street, was said by human rights group Safeguard Defenders to be one of 54 alleged clandestine bases run by the Communist state across five continents.

Investigative journalists at The Ferret reported in June that the Chinese Embassy in the UK claims it has now shut all of its secret police stations in Britain, after being told by the UK Government that they were “unacceptable”.

Safeguard Defenders claimed the secret police stations were set up by a Chinese state body to monitor dissidents and to influence foreign politicians.

An investigation by The Ferret for The Herald found that organisations linked to the alleged Chinese secret police station in Glasgow invited Nicola Sturgeon to the premises while she was first minister.

But Ms Sturgeon declined the invitation.  The paper reported that former First Minister Alex Salmond accepted an invitation to Loon Fung in 2014.

The Ferret previously revealed that the man it linked to Loon Fung, Jimmy Lin, met with Boris Johnson at a Conservative party fundraiser while he was Prime Minister.

The revelations prompted both the Inter-Parliamentary Alliance on China and the Scottish Liberal Democrats to call for an investigation into how Chinese state operatives may have attempted to influence political leaders.

Loon Fung has denied accusations of wrongdoing.