The Metropolitan Police have arrested a man on suspicion of arson with intent to endanger life after fires at homes linked to Prime Minister Sir Keir Starmer.
Police arrested the 21-year-old in the early hours of Tuesday. He remains in custody.
The arrest followed a fire at the prime minister's private home in Kentish Town, north London, in the early hours of Monday.
On Sunday, emergency services were called to a small fire at the front door of a house converted into flats in nearby Islington, a property also linked to Sir Keir.
Police are also looking at a car fire as part of the investigation.
No one was injured in the fires. The prime minister's official residence is in Downing Street.
Counter-terrorism police took over the investigation. Police are treating the fires as suspicious and investigating whether they are linked.
"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," the Metropolitan Police said.
The first fire under investigation happened just before 03:00 BST on Thursday, when firefighters were called to a small car fire on the same street as the prime minister's Kentish Town home.
The second occurred just after 03:00 on Sunday at the Islington property. One person was assisted to safety via an internal staircase by crews wearing breathing apparatus, London Fire Brigade (LFB) said.
The third took place at the Kentish Town property. LFB said it had been called to a "small fire" at 01:11 on Monday that was under control about 20 minutes later.
There was damage to the property's entrance but nobody was hurt, the Metropolitan Police said.
On Monday, Sir Keir thanked the emergency services for their work, his official spokesman said.
He added the matter was "subject to a live investigation so I can't comment further", and declined to provide any further details on who was in the property when the fire started.
Home Secretary Yvette Cooper told the BBC on Tuesday she had been briefed on the incidents but could not comment further.
"Police are investigating these incidents and they have my full support in doing so," she said.
"With any incident, there are always the important concerns that all of us will have, but we also have confidence in our police and the work that they are doing to investigate this incident."