The United States would enthusiastically support a no-deal Brexit if that is what the British government decided to do, US national security adviser John Bolton said on Monday during a visit to London aimed at reassuring Britain over UK-US ties.

Bolton told British Prime Minister Boris Johnson that President Donald Trump wants to see a successful British exit from the European Union on October 31 and that Washington will be ready to work fast on a US-UK free trade agreement.

Johnson wants the EU to renegotiate the terms of Britain's exit ahead of an October 31 departure date, but the EU says it will not alter the part of the deal that Johnson says must be changed.

The impasse leaves Britain facing an exit without any formal transition period or legal agreement covering issues such as trade, data transfers and border policy.

"If that's the decision of the British government we will support it enthusiastically, and that's what I'm trying to convey. We're with you, we're with you," Bolton told reporters after his first day of meetings.

As Britain prepares to leave the European Union, its biggest geopolitical shift since World War II, many diplomats expect London to become increasingly reliant on the US.

Bolton, in London for two days of talks, is seeking an improved US-British relationship with Johnson after sometimes tense ties between Trump and Johnson's predecessor, Theresa May.

He said British officials had given him an unmistakable sense that they were determined to honor the 2016 referendum vote to leave the EU. Bolton offered his support for this stance:

"The fashion in the European Union: When the people vote the wrong way from the way the elites want to go, it's to make the peasants vote again and again until they get it right," he said.

The central message Bolton was delivering is that the US would help cushion Britain's exit from the EU with a free trade deal that is being negotiated by US Trade Representative Robert Lighthizer and his British counterpart, Liz Truss.