The partner of a sitting Labour MP is among three men who have been arrested on suspicion of spying for China, the Guardian understands.

The Met Police took the men into custody on Wednesday morning on suspicion of assisting a foreign intelligence service, and as part of a wider investigation into national security offences related to China.

Detectives from counter-terrorism policing London, which is leading the investigation, arrested a 39-year-old man in London, a 68-year-old man in Powys, Wales, and a 43-year-old man in Pontyclun, Wales. The police have not identified the men, who remain in custody.

However, sources told the Guardian they included the partner of a Labour MP. A second person is understood to be the partner of a former Labour MP. The identity of the third, or whether they have any links to parliament, is unknown.

The police said the arrests were part of a “proactive investigation” and that there is not an “imminent or direct threat to the public”.

As well as searches at the addresses where the arrests were made, counter-terrorism detectives are carrying out searches at what it understood to be residential addresses in London, East Kilbride in Scotland, and Cardiff in Wales.

Security services have been investigating foreign interference in UK democracy, including attempts to obtain information on policymaking and interfere with sovereign affairs.

Previous cases of alleged spying within parliament have caused deep concern in Westminster, prompting warnings of a serious “systemic challenge” to British democracy.

More details soon …