On the eve of his UK state visit, US President Donald Trump vowed to “go all out” to swiftly secure a free trade deal between the UK and US within months of Britain exiting the EU.

US Ambassador to the UK Woody Johnson has revealed that Washington has “lined up” preparations for a post-Brexit trade deal with the UK and will be “ready to go” as soon as Britain resolves its exit from the EU.

Speaking on BBC One's Andrew Marr Show, Johnson asserted that “whatever happens” with Brexit, the US and UK will enjoy a “great relationship”.

The Ambassador claimed Donald Trump's office was currently negotiating with the UK trade department to secure a US-UK trade deal and deliver it in a speedy manner once Brexit is fixed. 

He said: “I think the fact that it’s on the President’s desk on day one, the minute that you leave, and we can negotiate.”

“We are already negotiating; we’re already looking at the terms of the conditions that will allow successful negotiations.”

“I think it will be done with the President looking at it, it will be done as expeditiously as any agreement we ever had.”

Woody Johnson said the “entire economy” would be part of any transatlantic agreement.

“We’re looking at all the components of the deal and trying to get everything lined up so that when the time comes, we’re ready to go,” concluded Johnson.

On the eve of his UK state visit, Donald Trump characteristically flouted diplomatic precedent as he waded into British politics and called on Britain to leave the European Union without a deal if Brussels refuses to meet its demands.

Trump added that if he was in charge, he would not pay the EU divorce bill, and he claimed it is not too late to “sue” the EU to give Britain greater “ammunition” in the talks.

He told the Sunday Times: “If I were them I wouldn't pay 50 billion dollars. That is me. I would not pay, that is a tremendous number.”

He also urged the Government to send Nigel Farage into the negotiations, stressing it was a “mistake” not to involve Brexit Party leader Nigel Farage in negotiations, saying he has a “lot to offer” and is someone he likes “a lot”.

Donald Trump has promised to “go all out” to ensure a free trade deal between the UK and US is delivered promptly within months of Britain exiting the EU.

The agenda for the first day of Donald Trump's state visit to the UK includes a private lunch with the Queen, tea at Clarence House with the Prince of Wales and the Duchess of Cornwall, and a state banquet at Buckingham Palace.