President Donald Trump should understand the U.K. will always be prepared to challenge his views in a respectful way — regardless of whether his feelings get hurt, London Mayor Sadiq Khan told CNBC Friday.

“The great thing about our city is you can have a fraternal and positive relationship with a country and its citizens but disagree in a mature and adult way with the president,” London Mayor Sadiq Khan told CNBC’s Wilfred Frost on Friday morning.

“One of the great things about having a best friend as we do with America is we can call you out when we think you’re wrong … I think we shouldn’t be afraid of doing so,” he added.

The U.S. president is currently visiting the U.K. as part of a working visit to meet with Prime Minister Theresa May, Queen Elizabeth II and several other business leaders. He is then expected to travel to Scotland for a weekend of golf.