Jacob Rees-Mogg said London mayor Sadiq Khan “demeans his office” and “the nation” by speaking ill of Donald Trump.
The London mayor recently called the US president a "20th century fascist", which the Conservative MP said was “quite wrong and deeply improper”.
“I think what the mayor of London said is deeply disgraceful. The fascists of the 1930’s massacred millions of people and Mr Trump is the democratically elected president of our closest ally," he told talkRADIO's Julia Hartley-Brewer.
The US president hit back at Mr Khan, calling him a “stone cold loser” as he landed in the UK for his three-day state visit.
But Mr Rees-Mogg said he is “backing Mr Trump in this row”, and believes it’s “perfectly reasonable” for the president to use his right of reply.
“For the president to hit back and say he’s a failed mayor of London which is true with spiralling knife crime, deeply congested roads, a general incompetence in city hall is fair enough,” he said.
"The security of the United Kingdom is closely tied in with our friendship with the United States”.
Sadid Khan said the country should not "roll out the red carpet" for Mr Trump, but Mr Rees-Mogg disagreed.
“Of course we should be rolling out the red carpet. It should be of the deepest red and the finest hue," he said.
Both Mr Trump and Mr Rees-Mogg have endorsed Boris Johnson in the race to replace outgoing Prime Minister Theresa May.
In a recent interview the president said "I think Boris would do a very good job.”
"I don't know that he is going to be chosen but I think he is a very good guy, a very talented person."
Mr Rees-Mogg said it’s “important” the Prime Minister has a working relationship with the US president.
“It is the most important relationship from this country’s point of view, and that Boris can from day one have a good relationship with president Trump is good news.”