The detached house on a suburban street in Wimbledon was an incongruous venue for a meeting about spies, secrets and the incoming president of the United States.

But it was here, at a £1.7 million South London home, that former MI6 agent Christopher Steele came a week after Donald Trump’s shock election victory in November 2016 on what he saw as a mission of national security.

For months Mr Steele, once in charge of MI6’s Russia desk but now approaching a decade out of the service, had been grappling with astonishing intelligence he had collated about the Trump campaign.

Contracted by a Washington DC-based research firm to investigate Mr Trump before the election, Mr Steele had put feelers out to around 20 sources about his Russia ties that summer and autumn.

What came back was explosive. Allegations of suspect meetings between Trump advisers and Russians, a Kremlin drive to tilt the election in Mr Trump’s favour, and, perhaps most worrying of all, alleged “kompromat” on the candidate himself.

With Mr Trump now heading for the White House for certain, Mr Steele decided it was his obligation - his national duty - to alert the British Government about what he had found.