A 19-year-old man from Harlow, Essex, was detained on Wednesday on suspicion of conspiracy to commit arson in connection with fires targeting properties and a vehicle linked to Sir Keir Starmer.
The teenager was taken into custody at an Essex police station before being released under investigation while enquiries continue, the Metropolitan Police confirmed.
He becomes the fifth person to be arrested as part of the ongoing probe into three separate blazes that occurred in north London during May last year.
All incidents under investigation have connections to the Prime Minister, prompting Counter Terrorism Policing London to lead the case.
The first incident occurred on May 8 when a Toyota RAV4, which the Prime Minister had previously sold, was set ablaze in Kentish Town.
Three days later, on May 11, flames engulfed the front entrance of a house converted into flats in Islington that was also connected to Sir Keir.
A day later, the most serious incident took place in the early hours, when fire broke out at the residential property where Sir Keir had lived before moving into Downing Street following his general election victory.
The London Fire Brigade alerted police to the blaze at approximately 1:35am, with crews bringing it under control within 20 minutes.
A fifth man has been arrested in an arson probe into a series of fires at properties linked to Sir Keir Starmer
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PAThree men have already been charged in connection with the fires and remain in custody awaiting trial.
Roman Lavrynovych, a 21-year-old Ukrainian national from Sydenham, faces three counts of arson with intent to endanger life.
Petro Pochynok, 34, also a Ukrainian national from north London, and Stanislav Carpiuc, a 26-year-old Romanian national from Romford, have each been charged with conspiracy to commit arson with intent to endanger life.
Mr Lavrynovych and Mr Pochynok both entered not guilty pleas at a hearing in October, while Mr Carpiuc pleaded not guilty to the same charge the following month.
Counter Terrorism Policing London is leading investigations into the fires
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PAA fourth man, aged 48, who was arrested at Stansted Airport in June last year on suspicion of conspiracy to commit arson with intent to endanger life, has since been released without charge.
Officers from Counter Terrorism Policing London are heading the investigation due to all three fires having connections to a high-profile public figure.
The three charged defendants are scheduled to appear at the Old Bailey on April 27 for trial.
Police have urged anyone with information that could assist investigators to get in touch.
This can be done by contacting the force on them on 101, quoting reference CAD 441/12MAY/2025.
Enquiries into the incidents remain ongoing, the Met added.