At a Downing Street press conference, Mr Johnson rejected Mr Trump’s claim that the result of November’s presidential election was fraudulent, saying he was pleased that “democracy has prevailed” through the confirmation of Joe Biden as president-elect.
Mr Johnson’s “unreserved” condemnation came after home secretary Priti Patel said that the outgoing president’s incendiary comments “directly led” to the violent scenes in Washington DC which shocked the world on Wednesday.
It marks the PM’s biggest effort yet to distance himself from the man who he once said had “many, many good qualities” and who in return described Johnson as “Britain Trump”.
It came after Labour branded the PM “spineless” for failing to call out Mr Trump’s lies about election fraud.
“The violence that Donald Trump has unleashed is terrifying, and the Republicans who stood by him have blood on their hands,” said Labour deputy leader Angela Rayner.
“Our spineless prime minister and toadying foreign secretary have to also take their fair share of shame for not calling out his lies after the election.”
At least 52 people were arrested after a mob of Trump supporters breached the Senate chamber as politicians were convening to formally certify Joe Biden’s presidential election victory – just 13 days before he enters the White House.
After rioters clashed with the police in scenes the former Republican president George Bush described as an “insurrection”, authorities confirmed that one woman died during the violence and that three others had also died after suffering “medical emergencies”.
In a tweet issued while the violent events were still unfolding in Washington on Wednesday, Mr Johnson described the riot as “disgraceful” but did not directly link it to Trump, posting: “The United States stands for democracy around the world and it is now vital that there should be a peaceful and orderly transfer of power.”
But asked at Thursday evening’s press conference whether he felt Mr Trump was responsible for the shocking scenes, the PM replied: “All my life America has stood for some very important things – an idea of freedom, an idea of democracy.
“And insofar as he encouraged people to storm the Capitol and insofar as the president consistently has cast doubt on the outcome of a free and fair election, I believe that was completely wrong.
“What President Trump has been saying about that has been completely wrong and I unreservedly condemn encouraging people to behave in the disgraceful way they did in the Capitol.
“I am very pleased that the president-elect has been duly confirmed in office and that democracy has prevailed.”
Speaking to BBC Breakfast earlier in the day, Ms Patel described the events as “tragic”, adding: “Well, his [Mr Trump’s] comments directly led to the violence and so far he has failed to condemn that violence – that is completely wrong.
“What we’ve seen in Washington and the display of the violence. He basically has made a number of comments yesterday that helped to fuel that violence and he didn’t do anything to de-escalate that whatsoever.”
Priti Patel condemns deadly riots at US Capitol
She added: “There is no justification for the violence. Donald Trump should have really condemned that violence and he’s failed to do that and I actually think that’s wrong.”
Pressed on whether she had any “respect” for the US president, Ms Patel replied: “His comments have been associated with that violence.
“He has failed to condemn that violence and I think that is completely wrong. People have died. What we have seen is just so extraordinary.
“There is no justification for it. Clearly there is more he could have done and should have done and the fact he has failed to condemn that violence is absolutely wrong.”
In a separate interview on BBC Radio 4’s Today programme, Ms Patel declined to say whether she regretted how close the Conservative Party had been to the US president in recent years.
She replied: “America is one of our closest allies in the world and they will continue to be, whoever is at the top in the White House. But the fact of the matter is right now this isn’t about our relationship with Donald Trump.
“They are now transitioning to a new president, to president-elect, the prime minister has already been in touch with Joe Biden and certainly congratulated him. On that basis alone, we move forward with one of our greatest allies in the world. This isn’t about going back and reflecting on personal relationships.”