The Good Morning Britain presenter called for Trump, who he also called a “monster”, to be removed from office as President of the United States.
It comes after a mob of pro-Trump rioters violently stormed the US Capitol in Washington DC on Wednesday, clashing with police as the heart of US democracy went into lockdown.
Trump had addressed thousands of supporters at a rally near the White House, egging them on to march to Capitol Hill, when rioters breached barricades and fought past police to storm into the building.
In a column about the events for MailOnline, Piers wrote: “In the past year, he has morphed into a monster that I no longer recognise as someone I considered to be a friend and thought I knew.
“I never imagined he would be capable of becoming what he’s now become – the deranged figurehead of a bunch of hard-right conspiracy-theory-fuelled fanatics intent on destroying American democracy.”
He added: “I can’t remain friends with someone who says he ‘loves’ a bunch of violent Nazi sympathisers and white supremacists who’ve just launched a deadly attack on the epicentre of US democracy itself and thinks they’re special people.”
Piers has been friends with Trump for 15 years, famously appearing opposite him on the US version of The Apprentice in 2007.
The pair fell out last year when the presenter criticised the American leader’s handling of the coronavirus crisis.
Piers later told This Morning that they’d buried the hatchet over the phone, pointing out: “I still consider him to be a friend, I’ve known him a long time.”
However, he has now called for Trump to be removed from office and should be impeached, which he noted would have the “additional benefit of barring him ever for running for the presidency again”.
“Such a person should not be President of the United States for a second longer,” he added.
Amid the riots, Trump called the rioters “very special” in a minute-long video on social media, telling them to go home.
He said: “I know your pain. I know you’re hurt. We had an election that was stolen from us... But you have to go home now. We have to have peace. We have to have law and order. We have to respect our great people in law and order...
“So go home, we love you, you’re very special. You’ve seen what happens, you’ve seen the way others are treated that are so bad and so evil. I know how you feel, but go home and go home in peace.”
President-elect Joe Biden blamed Trump for “inciting a mob to attack the Capitol to threaten elected representatives of the people of this nation, and even the vice president, to stop the Congress ratifying the will of the American people and a just-completed, free and fair election”.
Trump was “trying to use a mob to silence the voices of nearly 160 million Americans who summoned the courage in the face of a pandemic that threatened their health and their lives and cast that sacred ballot”, Biden added.
Boris Johnson has also said Trump was “completely wrong” to cast doubt on the US elections and rally people to storm Capitol Hill.
After Trump finally conceded to Biden for the first time on Thursday evening, he condemned the violence, saying rioters had defiled the seat of American democracy and said his focus would now turn to ensuring a smooth transition.
Trump did not address his role in inciting the violence in his video.