Karl MercerLondon political editor
PA Media
Sir Sadiq Khan said Donald Trump's comments at the UN were "racist, sexist, misogynistic and Islamophobic"
The prime minister has described claims from the US president that London wants "to go to Sharia law" as "ridiculous nonsense".
Sir Keir Starmer also said he supported Mayor Sir Sadiq Khan following Donald Trump's statement to the UN General Assembly in New York on Tuesday.
Trump had told the meeting that the city had a "terrible mayor" and had been "changed".
He said: "Now they want to go to Sharia law. But you are in a different country, you can't do that," which Sir Sadiq said showed Trump was "racist, sexist, misogynistic and Islamophobic".
EPA
Sir Keir Starmer said Donald Trump's statement to the UN was "nonsense"
Sir Keir said: "I'm not going to get drawn into a war of words, but what I will say is this, because it is important.
"You saw from the state visit last week that there are plenty of things on which the president and I agree, and we are working together.
"There are some issues on which we disagree, and what the president said about the mayor, who's doing a really good job, in fact driving down serious crime, what he said about the introduction of Sharia law was ridiculous nonsense.
"I support our mayor, I'm really proud of the fact we have a Muslim mayor of a very diverse city.
"We do work with the Americans on a huge number of issues. On this issue I disagree, and I stand with our mayor."
He added that he had spoken personally to Sir Sadiq about Trump's claims.
Sir Sadiq added that he appeared to be "living rent-free inside Donald Trump's head".
Responding to the president calling him a "terrible mayor", he said he was "thankful we have record numbers of Americans coming to London".
He said different criteria showed London was "often the number one city on the globe when it comes to culture".
Watch: President Trump says he's "not a fan" of Sir Sadiq Khan
Cabinet minister Pat McFadden rejected the claim that London wanted to "go to" Sharia and said British law and "no other kind of law" applied in the UK.
Earlier this month, justice minister Sarah Sackman told the House of Commons Sharia formed "no part of the law of England and Wales".
"Where people choose to put themselves before those councils, in common with Christian, Jewish and other courts of faith, that is part of religious tolerance which is an important British value," she said.
While religious courts, including Sharia councils, do operate in the UK, most of their work deals with religious marriage arbitration and financial matters.
The government has been clear that their rulings are not legally binding.