US President Donald Trump says he intends to run for re-election in 2020 because "everybody wants me to" and there are no Democratic candidates who could defeat him.

Asked by British journalist Piers Morgan in an interview whether he was going to run Trump said he fully intends to.

In the 30 minute long interview, Trump also revealed details of his meeting with the Queen.

The President described the Queen as "fantastic" and as "beautiful inside and out".

Before leaving Britain for a summit in Finland with Russian President Vladimir Putin, Trump breached British royal protocol by publicly disclosing the details of a conversation he had with the Queen about the complexities of Brexit.

When asked if he discussed Brexit with the monarch when they met for tea at Windsor Castle on Friday, Trump said: "I did."

"She said it's a very - and she's right - it's a very complex problem."

On a visit to the Queen with her husband, then First Lady Michelle Obama also broke protocol when she put her arm around the Queen's back.