The Archbishop of Canterbury Justin Welby says he was "shocked" by the way that British Prime Minister Boris Johnson dismissed criticism of his language in the contentious Brexit debate.

Johnson dismissed warnings about the tone of language used around Brexit, including death threats against MPs, as "humbug". 

In an interview with The Sunday Times, Welby said that with society being so "polarised and volatile" over the question of Britain's withdrawal from the European Union, it was "extraordinarily dangerous to use careless comments". 

Asked what he made of Johnson's "humbug" comment, the Archbishop said: "I was shocked by that . . . It should never be dismissed in that way.

"Death threats are really serious and they need to be taken seriously. All sides need to say, 'That is totally and utterly unacceptable.'" 

He said that politicians needed to be even more cautious in the modern era than past leaders like Winston Churchill because social media "amplifies things" 

"Churchill was well known for his somewhat inflammatory putdowns in parliament," he said.

"But this is happening at a time when we have social media, which amplifies things. In a time of deep uncertainty, a much smaller amount of petrol is a much more dangerous thing than it was in a time when people were secure.

"There is a great danger to doing it when we're already in a very polarised and volatile situation."

He added that society needed to move away from an "abusive and binary approach to political decisions" where people have the attitude of: "It's either this or you're my total enemy."

Welby gave the interview during a visit to the Democratic Republic of Congo where he was visiting people affected by a deadly outbreak of Ebola. 

At least 2,000 people have died since the outbreak of the disease in August 2018.  After visiting Ebola treatment centres, he spoke candidly about what he had seen, saying: "There is hope but there are bits that are pretty horrifying."