Johnson's Brexit deal with the EU, saying it restricts the US ability to do future trade with the UK. Speaking to LBC, he said that, without the deal, the two countries could "do many times the numbers" than now, the BBC reported on Thursday.
The US President also took a swipe at Labour leader Jeremy Corbyn, saying he would be "so bad" as prime minister. Corbyn accused him of "trying to interfere" in the UK general election to boost "his friend Boris Johnson".
The UK is officially going to the polls on December 12 after the early election bill became law when it was given royal assent on Thursday.