On Monday’s Good Morning Britain, Piers said he feels the president was ‘sent nuts’ by losing the election and regretted supporting him in the past.
Their friendship has recently broken down with Trump unfollowing the TV star on social media.
The presenter said: ‘Horrendous, what we witnessed in America last week was one of the saddest thing I have ever had to see, in a country I love.
‘It was a mob of insurrectionists attempting a coup, encouraged to fight down at the Capitol, by their own President.
Piers Morgan is ‘done’ with President Donald Trump (Picture: Getty)
‘I’ve been saying this for a while, Trump has gone mad. I think the pandemic, and the realisation he lost the election, have sent him nuts.’
Piers continued: ‘To everyone who gave him the benefit of the doubt, including me for a long time, there’s no doubt I never ever imagined the person I’d known for 15 years would be capable of inciting a mob to attack the Capitol and attack democracy itself.
‘He didn’t just cross the line he trampled all over that line and he’s now a threat to American democracy.’
As his co-host Susanna Reid explained the president could be ‘impeached or removed’, Piers replied: ‘He could be impeached if he is deemed to be mentally unfit, and I think there is a good argument for that.’
He added that Trump had ‘culpability’ for ‘whipping that mob up for weeks into believing the falsehood that this election was stolen from him when it wasn’t.’
Piers previously interviewed President Trump in 2018 where they discussed topics such as his – since suspended – Twitter account, gun control, Brexit and climate change.
Good Morning Britain airs weekdays from 6am on ITV.