A researcher at the UK Parliament has been arrested under the Official Secrets Act, amid claims he was spying for China.
Police have confirmed two men, one in his 20s and another in his 30s, were arrested under the act in March.
Sources have told the BBC one of them was a parliamentary researcher involved in international affairs issues.
As first reported in the Sunday Times, it is understood the researcher had links to several Conservative MPs.
On Sunday morning, No 10 said Rishi Sunak had expressed concerns about Chinese interference to a senior official from China.
A spokesperson said the prime minister had met Chinese Premier Li Qiang during the G20 summit in India, and "conveyed his significant concerns about Chinese interference the UK's parliamentary democracy".
Chinese President Xi Jinping is not attending the summit.
The Sunday Times reported the researcher had links to security minister Tom Tugendhat and foreign affairs committee chairwoman Alicia Kearns, among others.
Several government sources declined to comment on security matters.
The Metropolitan Police said: "A man in his 30s was arrested at an address in Oxfordshire and a man in his 20s was arrested at an address in Edinburgh.
"Searches were also carried out at both the residential properties, as well as at a third address in east London."