British Prime Minister Theresa May's government said she was looking forward to discussing her Brexit strategy with US President Donald Trump when the two meet for lunch on Friday after the US leader criticized her plans for withdrawing from the EU in an interview.

In the wide-ranging interview with British tabloid The Sun, Trump also slammed London Mayor Sadiq Khan and claimed that crime was rising in the capital, and suggested that former Foreign Secretary Boris Johnson would be a good prime minister.

"I would have done it much differently. I actually told Theresa May how to do it but she didn't agree, she didn't listen to me," Trump said of Brexit negotiations, adding that they were going "too slow."

"When a deal takes so long, they never work out very well," Trump said hours before meeting May for dinner. He added, however, that he thought the PM was a "very good person."

Later, at a joint press conference at the prime minister's residence outside London, Trump told reporters that he had also praised May in the interview, but that that part had been omitted. It was, he said, a case of "fake news." 

Trump also said that if May's Brexit plans included sticking to EU rules, that could prevent a US-UK trade deal.

"If they do a deal like that, we would be dealing with the European Union instead of dealing with the UK, so it will probably kill the deal," Trump said, adding that "we have enough difficulty with the European Union."

Many Brexit supporters used the opportunity for the UK to negotiate its own trade agreements as the basis for their call to leave the EU.

Trump 'saddened' by Johnson's resignation

The Sun then asked about Brexit supporter Johnson, who resigned as the UK's top diplomat earlier this week over a spat with May about how to handle leaving the European Union.

"I was very saddened to see he was leaving government and I hope he goes back in at some point," Trump said. "I think he is a great representative for your country."

Though he added that he thought Johnson would one day be a good candidate to lead the country, he insisted that he wasn't trying to put Johnson against May.

Khan 'has done a bad job'

The president did not have such glowing words for London's mayor.

Trump claimed that Khan has done a "very bad job on terrorism," as well as on crime. Two years ago, Khan first crossed Trump's radar by criticizing his call for a ban on Muslim immigration to the US.

"I think he has done a bad job on crime, if you look, all of the horrible things going on there, with all of the crime that is being brought in," Trump said of Khan.

Trump also told The Sun that he "feels unwelcome" in London because of the thousands of people expected to protest in the British capital, as well as other cities. 

In the Evening Standard newspaper, Khan wrote: "The planned protests are not anti-American but to show that we support hope, unity, tolerance and equality."

Immigration 'changing fabric of Europe'

The president also took the opportunity to express his "great love for countries in Europe," but claimed that allowing in refugees fleeing conflict and famine was causing the continent to lose its culture.

"Allowing the immigration to take place in Europe is a shame," he said. "I think it changed the fabric of Europe and, unless you act very quickly, it's never going to be what it was, and I don't mean that in a positive way."

The Sun is no stranger to publishing controversial material. Earlier in July, it was the subject of an official complaint by the Colombian ambassador the UK after it published a headline saying the only important things from Colombia were coffee, Shakira and cocaine ahead of a World Cup match.

Trump arrived in the UK on Thursday for a four-day visit, including two days at his Scottish golf resort in Turnberry and a property in Aberdeen.