Opponents of Donald Trump must continue calling the US President a liar, Bernie Sanders has said.

The veteran Democrat defended his use of the word after a Washington Post columnist, Amber Phillips, accused him of lowering the quality of US political discourse.

In a post on Medium, Mr Sanders wrote: “What should a United States senator, or any citizen, do if the president is a liar? Does ignoring this reality benefit the American people?

“Do we make a bad situation worse by disrespecting the president of the United States? Or do we have an obligation to say that he is a liar to protect America’s standing in the world and people’s trust in our institutions?”

The 75-year-old said he believed “strongly in civil political discourse” but said tough language was necessary because Donald Trump has “complete disregard for reality and makes assertions heard by billions of people around the world that have no basis in fact”.

The Democrat, who ran against Hillary Clinton for his party’s presidential nomination, has previously accused Mr Trump of being a “pathological liar” who “changes his mind every single day”.

In his latest post, he highlighted apparent mistruths including Mr Trump’s claim that his victory was the biggest electoral college win since Ronald Reagan. In truth, Barack Obama won by a greater margin in 2012.

The Vermont senator also mentioned Mr Trump’s unsubstantiated allegation that up to five million people had voted illegally in the presidential election.

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Mr Sanders said he had “great concern” that the Republican’s tendency to “lie over and over again” will undermine the credibility of the US and affect America’s ability to bring countries together to respond to crises, because nations “will think that Trump is just lying one more time”.

He also criticised the Republican over his claims Barack Obama ordered telephones in Trump Tower to be tapped during the election campaign.

In a series of early-morning tweets on Saturday, Mr Trump wrote: "Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism!

“How low has President Obama gone to tapp [sic] my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!”

He did not provide any evidence for the allegations and several of his spokespeople have since refused to do so.