The fires in May 2025 damaged the home Starmer moved out of when he became prime minister, as well as an apartment building he once owned a share of and destroyed his former Toyota SUV.
Nobody was injured in the blazes.
Ukrainian national Roman Lavrynovych, 22, and Stanislav Carpiuc, a 27-year-old Romanian citizen, were found guilty of a conspiracy to damage property by fire. Petro Pochynok, 35, was acquitted of the charge.
Lavrynovych was also convicted of two counts of committing arson with intent to endanger life or recklessly endangering life.
A Russian-speaking figure who went by the name "El Money" hatched the scheme and offered Lavrynovych money via the Telegram messaging app to torch the properties and get video of the evidence that could be posted online to draw attention to the attack.
El Money's identity was never revealed and he was not charged. The court heard how he gave Lavrynovych detailed instructions on the targets, how to mix flammable substances and steps to avoid being caught.
There was no evidence proving a hostile state orchestrated the fires because UK police did not discover the motivations or people El Money worked for, said Commander Helen Flanagan, head of the counterterrorism team at the Metropolitan Police.
"Clearly the tasking was to intimidate and create fear for the prime minister and to attack the UK," she said.
Lavrynovych said he needed money and admitted he set the fires, saying he wanted the bounty of STG3000 ($A5,695) in cryptocurrency to pay for medical treatment his father needed.
But he said he only followed through because he had been threatened by El Money.
Lavrynovych said he had no idea who owned the property until after the fires and said he didn't intend to hurt anyone.
After the fires, El Money promised Lavrynovych he would pay and told the Ukrainian to send him a secret message with the code word "geranium" if he was detained by police. Shortly after Lavrynovych received the message he was detained by police.
The court was told he never received any money for setting the three fires.
Carpiuc acted as a middleman, and Pochynok allegedly was recruited to record video of the fires so Lavrynovych could get paid.
The fires were set in the dead of night and occupants sleeping in the homes awoke to smoke billowing in their front doors.
Starmer's sister-in-law, who was living in his home, heard a loud bang and said she struggled to breathe as smoke filled a stairway. Her nine-year-old daughter was terrified.
An occupant of the apartment building retreated to the roof after discovering hallways full of smoke.
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