"I will not change my mind. I will not back down. It is not in our national interest to go into this war, and we will not do it," the prime minister said, according to the BBC.
He is referring to a war with Iran and Trump's new statement on Tuesday, in which the US president threatened to tear up the trade agreement between the countries.
"It's not our war. I've had a lot of pressure on me to take a different line, including what happened yesterday," Starmer said.
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