Donald Trump has launched an extraordinary attack on Theresa May in a scathing interview with the British press.

Speaking with The Sun's political editor Tom Newton Dunn, Mr Trump slammed the British prime minister over her Brexit plan, negotiation skills and trade stance, even saying her political nemesis Boris Johnson — who resigned earlier this week — would "make a great Prime Minister".

The interview couldn't come at a worse time for Ms May, who is currently in a vulnerable political position.

READ MORE: Diplomatic expert: Trump's foreign policy is unpredictable

Her proposal for the United Kingdom's departure from the European Union — which the majority of Brits voted in favour of in 2016 — has been deemed too "soft" by the movement's more conservative supporters.

Three senior government ministers recently quit over the issue. Mr Johnson sensationally quit on Monday after telling Ms May the Brexit "dream was dying", expressing his fears the UK would be reduced to a "colony" status.

Brexit Minister David Davis and minister for the Department for Exiting the EU Steve Baker also quit the government in protest.

No leadership challenge has emerged yet, but several British media reports suggest backbenchers may be waiting until Mr Trump's four-day visit concludes to demand that she resigns.

And this interview could be the thing to push them over the edge.

ON A 'SOFT BREXIT'

In the exclusive interview, Mr Trump warned that the British prime minister's soft Brexit blueprint will "kill" any future trade deal with the US.

To clarify — a "soft" Brexit is favoured by those who don't want Britain to leave the European Union, or to at least retain some links.

The UK would give up its seat on the European Council, but there could still be some sort of free movement of goods and services — and possibly people.

"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," Mr Trump told the The Sun.

Pro-Brexiteers favour a "hard" Brexit — which would essentially see the UK become totally independent. The UK would leave the single market and customs union, London would again full control of its borders, and cut money being sent to Brussels, meaning the country could negotiate its own independent trade deals.

Ms May's plan is essentially a compromise — the UK will seek a "free-trade area" with the EU for industrial and agricultural goods, maintaining its access to EU markets with a "common rule book" between the countries.

Critics of this agreement — including the ministers who quit — argue that it doesn't go far enough. "It seems to me we're giving too much away, too easily," Mr Davis said.

ON BREXIT ADVICE

Mr Trump claimed Ms May ignored his advice on Brexit negotiations, saying: "I would have done it much differently. I actually told Theresa May how to do it, but she didn't listen to me."

When asked if he would be prepared to walk away from the negotiating table, he responded: "Oh, absolutely. I think what is going on is very unfortunate. Too long.

"You know, deals that take too long are never good ones. When a deal takes so long, they never work out very well."

"The deal she is striking is a much ­different deal than the one the people voted on.

"It was not the deal that was in the referendum. I have just been hearing this over the last three days. I know they have had a lot of resignations. So a lot of people don't like it."

This isn't the first time Mr Trump has slammed Ms May's Brexit plan.

At a press conference in Brussels yesterday at the end of the NATO summit, he said: "I don't know if that's what they voted for."

A shocked Ms May responded: "We have come to an agreement at the proposal we're putting to the European Union which absolutely delivers on the Brexit people voted for. They voted for us to take back control of our money, our law and our borders and that's exactly what we will do."

ON PREDICTING BREXIT

Mr Trump even claimed to have predicted Britain's surprise vote to leave the European Union in 2016.

"I predicted Brexit," he said. "I was cutting a ribbon for the opening of Turnberry — you know they totally did a whole renovation, it is beautiful — the day before the Brexit vote.

"I said, 'Brexit will happen'. The vote is going to go positive, because people don't want to be faced with the horrible immigration problems that they are being faced with in other countries.

"You remember that Barack Obama said that there is no way it is going to happen, and the UK will get to the back of the line if it ever does, right? I said Brexit will happen, and I was right."

Despite his harsh criticism of Ms May's leadership, Mr Trump noted he "gets along with her nicely" and thinks she is "a nice person".

ON IMMIGRATION

The topic of immigration is more relevant than ever in light of Ms May's Brexit plan.

Under her proposal, immigration will come under control with the free movement of labour ending.

But it will be replaced with a mobility framework, and Ms May has refused to rule out giving EU citizens preferential rights after Brexit is pushed through — which has alarmed those who voted for it.

Mr Trump's opposition to immigration is no secret.

He argued Britain and the rest of Europe is "losing its culture" because of migrants and refugees, saying the wave of people coming from the Middle East and Africa is having a negative effect on the continent.

"I think what has happened to Europe is a shame.

"Allowing the immigration to take place in Europe is a shame.

"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.

"So I think allowing millions and millions of people to come into Europe is very, very sad.

"I think you are losing your culture. Look around. You go through certain areas that didn't exist 10 or 15 years ago."

Mr Trump also attacked London Mayor Sadiq Khan, suggesting he's responsible for the recent terror attacks on the city.

He argues the Labour figure has "done a very bad job on terrorism" by allowing so many migrants to come to the city.

The pair have had a long-running feud, with Mr Khan slamming Mr Trump's controversial ban on immigrants from six Muslim-majority countries.

ON THE NATO SUMMIT

Mr Trump launched an all-out attack during a two-day NATO summit held earlier this week, saying other countries aren't pulling their weight with their defence spending.

The US leader created panic in Europe as he demanded other NATO nations massively increase their military spending, triggering an emergency budget meeting.

He also claimed that the other NATO countries owe the US "a tremendous amount of money", arguing that the US is "paying a lot of money to protect" other countries in exchange for nothing — two claims which have been disproved.

While the United Kingdom is one of the few countries to exceed NATO's minimum target of two per cent of GDP on defence, Mr Trump believes this is not enough.

"Two per cent isn't enough. The US pays 4.2 per cent of a much larger GDP."

He dismissed accusations of blackmail over his threat to pull out of NATO if the other countries don't meet their targets — a threat which, if carried out, would likely see the organisation collapse.

"I'll tell you what, we've had 40 years of presidents saying the same thing in a nicer way and they got nothing, so call it what you want," Mr Trump said.

"They're taking ad­vantage of the United States. I'm not going to let it happen."

ON THE PROTESTERS

Mr Trump's arrival into the country was marred by furious protests, with thousands of people marching and holding signs outside the US Ambassador's official residence and Blenheim Palace.

Hundreds of protesters have also gathered outside at Regents Park where Mr Trump is to spend the night.

Extra security was put in place shortly after he touched down, but the leader was whisked straight into Blenheim Palace for a black-tie dinner without incident.

Earlier, Mr Trump told reporters: "I think they like me a lot in the UK, I think they agree with me on immigration.

"You see, what's going on throughout the world with immigration … I think that's why Brexit happened."

But in the interview, he said he feels "unwelcome" in London. He even admitted he was aware of the 20-foot "Trump Baby" blimp flying above Parliament Square.

"I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London.

"I used to love London as a city. I haven't been there in a long time. But when they make you feel unwelcome, why would I stay there?"

ut he still reckons if there was an "honest poll", his numbers in Britain would be "very strong".

"They want the same thing I want. I love the UK."

A larger protest against the President is planned in central London tomorrow.