A third man has been arrested on suspicion of arson over attacks on two properties and a car linked to Sir Keir Starmer.
The 34-year-old was held in Chelsea, south-west London, on suspicion of conspiracy to commit arson with intent to endanger life on Monday, the Metropolitan Police said.
His arrest follows that of a 26-year-old man, who was detained on suspicion of the same offence on Saturday at Luton Airport.
Roman Lavrynovych, a 21-year-old Ukrainian man, has already been charged with three counts of arson with intent to endanger life in connection with the fires.
Lavrynovych, of Sydenham, south-east London, denied arson in a police interview.
He appeared in court on Friday and was remanded in custody until a further hearing at the Old Bailey scheduled for June 6.
Two of the fires took place in Kentish Town – one in the early hours of May 12 at the home where Sir Keir lived before he became Prime Minister and moved into Downing Street.
A car was set alight in the same street four days earlier on May 8.
The third fire was on May 11 at the front door of a house converted into flats in Islington.
