The 48-year-old man was detained at London Stansted Airport on Monday and is being questioned on suspicion of conspiracy to commit arson with intent to endanger life, the Metropolitan Police said.
Three other men have been charged over the fires: Ukrainians Petro Pochynok, 34, and Roman Lavrynovych, 21, and Ukrainian-born Romanian national Stanislav Carpiuc, 26.
Lavrynovych is charged with three counts of arson with intent to endanger life, while Pochynok and Carpiuc are accused of conspiring to commit arson with intent to endanger life. They are due to appear at the Old Bailey on Friday.
It comes after a fire in the early hours of 12 May at a house in Kentish Town, north London, where Sir Keir lived with his family before the election.
Damage was caused to the property’s entrance, but nobody was hurt.
A car was also set alight in the same street on 8 May.
Another fire took place on 11 May at the front door of a house linked to Sir Keir that was converted into flats in Islington. One person was taken to safety via an internal staircase by crews wearing breathing apparatus.
