Police are continuing their investigation into alleged arson attacks on properties and a car linked to Sir Keir Starmer as a 21-year-old suspect remained in custody.
Counter-terrorism officers are working “at pace” to establish the cause of the fires and “any potential motivation”, Scotland Yard said.
The force said a 21-year-old was arrested at an address in Sydenham, south-east London, in the early hours of Tuesday on suspicion of arson with intent to endanger life.
He remained in custody at a London police station as of Tuesday evening.
In the early hours of Monday, the emergency services responded to a fire at the Kentish Town home where Sir Keir lived before becoming Prime Minister and moving into 10 Downing Street.
The property is understood to have been rented out to his sister-in-law since the Labour leader’s election success last year.
Counter-terrorism officers are leading the investigation into the fire on Monday “as a precaution and due to the property having previous connections with a high-profile public figure”, the Metropolitan Police said.
A fire at a property on Sunday and a car, also linked to Sir Keir, which was set alight on May 8 also form part of the probe.
Commander Dominic Murphy, head of the Met’s counter-terrorism command, acknowledged the probe may cause concern to MPs.
“I would encourage any MP who is concerned about their own safety to get in touch with their dedicated local Operation Bridger officer, who can provide further advice and support,” he said in a statement on Tuesday.
On Monday, a police cordon and officers, as well as investigators from London Fire Brigade (LFB), could be seen outside the Kentish Town property where the Prime Minister used to live.
The Met said residents could expect an increased police presence in the coming days.
Neighbours described hearing a loud bang and said police officers were looking for a projectile.
Charles Grant, 66, told reporters: “Police searched my garden and said they were looking for something somebody had thrown but they didn’t find anything. They said they were looking for a projectile.”
Police were alerted by the LFB to reports of a fire at the residential address at 1.35am.
Damage was caused to the property’s entrance but nobody was hurt.
In the early hours of Sunday, firefighters dealt with a small fire at the front door of a house converted into flats in nearby Islington, which is also linked to Sir Keir.
One person was assisted to safety via an internal staircase by crews wearing breathing apparatus, LFB said.
The car fire broke out in the early hours of Thursday May 8, in the same street as the Kentish Town property.
Downing Street said the Prime Minister was “focused on the job at hand” as the investigation continues, adding that police should be given the “time and space” to complete their inquiries.
Conservative leader Kemi Badenoch said her thoughts were with the Prime Minister and his family and warned that threats to people in public service are “an attack on our democracy and must never be tolerated”.
