Sir Keir Starmer's house in north London that was "firebombed" was being rented to his sister-in-law, it has emerged.
The four bedroom town house was reportedly doused in an accelerant and set light in the early hours of Monday, sparking a probe by counter-terror police.
A 21-year-old suspect was arrested at an address in Sydenham in the early hours of Tuesday on suspicion of arson with intent to endanger life.
The Telegraph reported that that since entering 10 Downing Street last summer, Sir Keir's sister-in-law has been living there on a peppercorn rent.
It is not known if she was at home at the time of the attack. The Prime Minister's official spokesman declined to comment on Tuesday.
The rental agreement is declared through the ministerial register of interests — but because the rent is below the £10,000-a-year threshold, it is not on the MPs’ register, the Mail reported.
It has also emerged a vehicle targeted in a separate attack on Thursday was reportedly a 4x4 that used to belong to Sir Keir's family and was sold to a neighbour.
Video and pictures show the car bursting into flames.
The two incidents have been linked to a third fire at a property on Sunday in Islington that has been linked to Sir Keir.
One person was helped from the property following a small fire at the front door of a house converted into flats on Sunday, London Fire Brigade said.
The man, 21, arrested by police has been held on suspicion of starting all three fires.
Downing Street said the police should be given the "time and space" to complete their investigations.
The Prime Minister's official spokesman said the security of MPs was taken seriously but stressed that Sir Keir Starmer was carrying on with his work.
The spokesman said: "He'll be updated in the usual way, you'll have seen the Met's statement.
"But the Prime Minister has chaired Cabinet this morning, he is focused on the job at hand, focused on delivering the Government's agenda."
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.
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.
"From what other people have told me today, I gather someone threw a firebomb at Keir Starmer's house."
A next-door neighbour, who didn’t want to be named, told the Sun he was awoken in the early hours of the morning to a loud bang.
“It was a loud bang, what sounded like glass shattering. It was scary,” he said.
Another resident, who asked not be named, said she was woken by a bang around 2am this morning.
The tech worker, 58, told the publication: “I got woken by a loud noise in the early hours of the morning.
“I looked out of the window and saw flashing lights and heard police sirens.
“There’s been lots of forensics outside all day. It seems like they’re taking it very seriously.”
Police were alerted by the LFB to reports of a fire at the residential address at 1.35am on Sunday
Damage was caused to the property's entrance but nobody was hurt.
Police said on Tuesday as they announced the arrest of a suspect: “A man has been arrested in connection with a series of arson attacks.
“The 21-year-old was arrested in the early hours of Tuesday, 13 May on suspicion of arson with intent to endanger life.
“He remains in custody.
“The arrest relates to three incidents.
“On Monday, 12 May at 01:35hrs, police were alerted by the London Fire Brigade to reports of a fire at a residential address in NW5.
“Officers attended the scene. Damage was caused to the property’s entrance, nobody was hurt.
“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.
“The investigation team are also considering two other incidents - a vehicle fire in NW5 on Thursday, 8 May and a fire at the entrance of a property in N7 on Sunday, 11 May - and are investigating whether they may be linked to the fire in NW5 on 12 May.
“All three fires are being treated as suspicious at this time, and enquiries remain ongoing.”
The suspect remained in custody on Tuesday evening.
Conservative leader Kemi Badenoch said: "This is a shocking incident. My thoughts are with the Prime Minister and his family. No one should face these sorts of threats, let alone people in public service.
"It's an attack on our democracy and must never be tolerated."
Sir Keir’s house has attracted protests in the past.
Last year, three pro-Palestinian activists were convicted of public order offences after unfurling a banner covered in red handprints outside the house.
