"This is not Winston Churchill that we're dealing with," Trump said at the White House, referencing Starmer's legendary World War Two predecessor.
The Oval Office comments on Tuesday were Trump's third broadside against Starmer this week as Washington's campaign of air strikes against Iran stoked concerns among some US partners who see the war as reckless and a violation of international law.
Trump and his aides have often scolded European allies over their immigration policies, lower-than-pledged military spending and hostility to far-right movements.
And Trump's often tepid support for Ukraine and his threats to seize Danish territory have raised fears in Europe about the stability of a transatlantic alliance facing growing threats from Russia.
Trump now says the US will cut off all trade with Spain after the European country refused to let the US military use its bases for missions linked to strikes on Iran.
"Spain has been terrible," Trump told reporters during a meeting with German Chancellor Friedrich Merz, adding that he had told Treasury Secretary Scott Bessent to "cut off all dealings" with Spain.
"We're going to cut off all trade with Spain. We don't want anything to do with Spain," he added.
Starmer has said Britain didn't take part in the US-Israeli assault on Tehran because any British military action must have a "viable, thought-through plan" and he did not believe in "regime change from the skies".
But he has since allowed the US to use UK bases to launch what he called limited and defensive strikes to weaken Tehran's capabilities, after Iran hit US allies in the region with drones and missiles.
On Monday, a British base in Cyprus was hit by a drone that Cypriot officials said was likely launched by Iran-backed Lebanese group Hezbollah.
During a White House meeting with German Chancellor Friedrich Merz, Trump vented frustration that the US had not been able to land its military jets at Britain's strategically important air base, Diego Garcia.
"I'm not happy with the UK," Trump said, unprompted, during a portion of the meeting that was open to the press.
"It's taken three, four days for us to work out where we can land. It would have been much more convenient landing there as opposed to flying many extra hours. So we are very surprised."
Trump had told the Telegraph on Monday that Starmer had appeared to be "worried about the legality" of the strikes on Iran.
Despite Trump's fraught relations with Europe overall, the Republican US president and the center-left Labour leader had until recently maintained upbeat personal ties.
Britain has for decades prided itself on its relationship with the US, aided by leaders such as Churchill, Margaret Thatcher and Tony Blair cultivating strong relationships with their counterparts, Franklin D. Roosevelt, Ronald Reagan and George W. Bush.
The two countries' "special relationship" spans intelligence sharing and military co-ordination.
"It's very sad to see that the relationship is obviously not what it was," Trump told the Sun newspaper in an interview published on Tuesday. He added that he never thought he would see Britain become a reluctant partner, and instead heaped praise on France and Germany.
Britain, France and Germany all released a joint statement in response to the Iranian attacks on Saturday, saying they were in close contact with the US, Israel and partners in the region, and were calling for a resumption of negotiations.
