Rishi Sunak has encouraged MPs who fear they have been targeted by an alleged Chinese spy to go to the police.
After the arrest of a parliamentary researcher suspected of working for Beijing, No 10 said the government would "robustly defend our democracy" but must continue to engage with China.
But Downing Street said politicians with fears over their interactions with the individual should report them.
Asked if the prime minister would encourage MPs to go to the police, No 10 said: “If anyone had any concerns that they had been targeted in any way shape or form -and again I’m not pre judging this specific case - then yes of course.”
The researcher has said he is “completely innocent”, in a statement released by his lawyers.
The man denied being a “Chinese spy” and said he had spent his career trying to “educate others” about the “threats presented by the Chinese Communist Party”.
On Sunday Rishi Sunak confronted the Chinese premier, Li Qiang, over what he termed “unacceptable” interference in British democracy. But he is expected to face questions from MPs over the row when he gives a statement on the weekend’s G20 conference in the Commons later.
Downing Street has insisted it has full confidence in the security minister Tom Tugendhat, who had links with the researcher at the centre of the row.
Asked what reassurance it could provide to people who may feel the minister had been compromised amid reports that the man had access to him, the Prime Minister's official spokesman said: "Obviously we have robust processes in place and advice available.''
Downing Street refused to say whether any extra vetting was taking place within the civil service following the revelations nor whether checks had been strengthened after MI5 raised an alert last year warning that a suspected Chinese spy had engaged in "political interference activities".
"I can't get into our approach to security, what measures we do or do not take, in response to specific threats," the spokesman said.
And No10 refused to say whether Mr Sunak’s confrontation with Chinese premier Li Qiang was the first time the government had raised the issue with Beijing.
Downing Street said details relating to a "live investigation" could not be shared, including when ministers were made aware of the case.
The parliamentary staffer was arrested along with another man by officers on 13 March, on suspicion of spying for Beijing, the Sunday Times revealed. One of the men, in his 30s, was detained in Oxfordshire, while the other, in his 20s, was arrested in Edinburgh, Scotland Yard said.
Both were held on suspicion of offences under section one of the Official Secrets Act 1911, which punishes offences that are said to be “prejudicial to the safety or interests of the state”.
The researcher had worked for a foreign affairs committee member and had worked in parliament for several years, but is believed to have previously worked in China.
He is reportedly a “skilled networker” rubbing shoulders with ministers, holding regular drinks events and using a dating app, including attempting to organise a date with a political journalist from The Sun.
The arrest has put Mr Sunak under increasing pressure to take a tougher stance on China, including to officially designate the country a “threat”.
Downing Street said it would be wrong to “reduce” the UK’s approach to China to “just one word” amid pressure to designate Beijing a threat following the arrest of a parliamentary researcher over suspected spying.
The Prime Minister’s official spokesman said the Government would “robustly defend our democracy” but that the UK must continue with engage with China.
“We do not think it is right to reduce the approach to just one word given we need to take the opportunity to engage with China, not to just shout from the sidelines,” he said.
The spokesman also refused to say whether foreign secretary James Cleverly had known about the arrest before his recent trip to China.
