In a speech on Monday, the Lib Dem foreign affairs spokesman said Prime Minister Mr Johnson had chosen to align himself with “right-wing, authoritarian nationalists” on the world stage.
He said Mr Johnson was “following the Trump playbook”
Asked afterwards if he thought Mr Johnson was racist, Mr Umunna told PA: “I think the things that he has said and done are racist.
“Only he in his heart knows whether he is a racist.”
He went on: “And likewise with Jeremy Corbyn, Jeremy Corbyn has done things which, arguably, amount to anti-Jewish racism.
“And he has allowed anti-Jewish racism to proliferate in the party, the Labour Party, during his leadership.”
Yahoo News UK has approached the Conservative and Labour parties for comment.
Speaking in Watford, Mr Umunna, who is standing in the Cities of London and Westminster constituency, added he was not surprised by the “squeeze” the Lib Dems had received in the polls, but insisted voting for the party was the only way to stop a Tory majority.
Mr Johnson, meanwhile, was on the campaign trail on Monday in the marginal Brecon and Radnorshire constituency: won by the Lib Dems in a by-election earlier this year.
He sheared a sheep as he visited the Royal Welsh Winter Fair in Llanelwedd, Wales.
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Mr Johnson also served several beef and lamb baps at a food stall at the show, donning an apron as he asked customers: “Can I interest you in a lamb bap?”
He then inspected some prize cattle in the bull ring, but one black bull became agitated as the Prime Minister toured the arena and had to be walked away.