The suspect was arrested in the early hours of Tuesday on suspicion of arson with intent to endanger life, according to the Metropolitan Police.
He remains in custody.
Emergency services were called to fires at the doors of two homes in north London within 24 hours of each other – one just after 1.35am on Monday and the other on Sunday. Both properties are linked to Sir Keir.
Detectives were also checking a vehicle fire in NW5 last Thursday to see whether it is connected.
Damage was caused to the property’s entrance, nobody was hurt.
The prime minister is understood to still own the home and used to live there before he and his family moved into 10 Downing Street after Labour won last year’s general election. It is believed the property is being rented out.
In a statement, police said: “As a precaution and due to the property having previous connections with a high-profile public figure, officers from the Met’s Counter Terrorism Command are leading the investigation into this fire.
“Enquiries are ongoing to establish what caused it. All three fires are being treated as suspicious at this time, and enquiries remain ongoing.”
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