Rishi Sunak has said he "will not accept" Chinese interference in the UK's democracy, after it emerged a parliamentary researcher was arrested amid accusations he spied for China.
In a statement to MPs, the PM said he told Chinese Premier Li Qiang at the recent G20 summit that any attempted spying "will never be tolerated".
The Met Police confirmed on Saturday that two men were arrested under the Official Secrets Act in March.
The researcher denies the allegations.
The man said in a statement he felt "forced to respond" to accusations in the media.
China has rejected the allegations of spying, with foreign ministry spokesperson Mao Ning calling it "malicious slander".
House of Commons Speaker Sir Lindsay Hoyle has warned MPs against identifying the man - who is not being named by the BBC - using parliamentary privilege.
During a statement to the House of Commons on the G20 summit in India, Mr Sunak told MPs: "I have been emphatically clear in our engagement with China that we will not accept any interference in our democracy and parliamentary system.
"We will defend our democracy and our security.
"So I was emphatic with Premier Li that actions which seek to undermine British democracy are completely unacceptable and will never be tolerated."
Under questioning from Sir Keir Starmer, Mr Sunak said Foreign Secretary James Cleverly had also raised China's attempts to interfere with UK democracy on his recent visit to China.
Sir Keir said "incidents like this show the constant threats that we face".
In a separate statement, Deputy Prime Minister Oliver Dowden said the government were "reviewing" increasing checks on figures working for the Chinese government in the UK.
Several MPs called for China to be classed in the "enhanced tier" of the Foreign Influence Registration Scheme, introduced earlier this year.
Adding countries to this creates additional reporting requirements for China-linked organisations.
Senior Tory backbenchers, including former prime minster Liz Truss and ex-Tory leader Sir Iain Duncan Smith, have called for the government to officially designate China as a threat to the UK - a move so far resisted by ministers.
Speaking in the Commons, Mr Dowden said there was a "strong case to be made" for this, but the government was "currently reviewing" which countries to add to the registration scheme.
He added that ministers were "clear eyed" about the challenges posed by China, but added it was not realistic to "completely disengage" with the country.
News of the arrests was first reported in the Sunday Times, which claimed the researcher had links to several Conservative MPs, including Security Minister Tom Tugendhat and Foreign Affairs Committee chairwoman Alicia Kearns.
Mr Tugendhat is said to have had only limited contact with the man, and no dealings with him as a minister.
The arrest of the researcher has renewed a debate among MPs about whether the UK should take a stricter approach to China.
China is the UK's fourth largest trading partner, and British ministers regularly highlight the need to work with the country on big international issues such as tackling climate change.
But relations have soured in recent years over a series of issues, including threats to civil liberties in the former British colony of Hong Kong and China's support for Russia during the war in Ukraine.