Keir Starmer (Photo: EPA)

London police have arrested a fourth suspect in an investigation into a series of arsons linked to British Prime Minister Keir Starmer , Reuters reports .

A 48-year-old man was detained at London Stansted Airport under the Terrorism Act and later arrested on suspicion of conspiracy to commit arson with intent to endanger life.

Last month, there were three related fires – in a house in north London owned by Starmer, in a nearby house where he previously lived, and in a car that also previously belonged to him.

Authorities have so far charged three men – two Ukrainians and a Romanian – in connection with the fires. None of them have been charged under terrorism laws or the new National Security Law, which aims to combat the activities of hostile states.

Three men, aged between 21 and 34, have been taken into custody and are due to appear at the Old Bailey in London on June 6.