Paul Ryan, the US House speaker, has said America was ready to forge a new bilateral trade deal with the UK as soon as possible.
His commitment to a new economic relationship between the US and UK came almost a year to the day that Barack Obama, the former president, threatened to send Britain to the "back of the queue" if it voted to leave the European Union.
The House speaker said Brexit should be viewed as "a real opportunity for our two nations" and warned against China "writing the rules of the 21st century global economy".
Mr Ryan said he was in the UK to bring a message "that the United States stands ready to forge a new trade agreement with Great Britain as soon as possible, so that we may further tap into the great potential between our people".
"As soon as the UK is ready, we will stike a bilateral trade agreement," Mr Ryan said.
He offered the unequivocal support of America as Britain attempts to secure deal with the European Union for how Brexit will work.
"We will stand with you through all of it," he said during a speech at the Policy Exchange think tank in London. "We're going to be with you through every step of the way."
He compared Brexit with the shock election victory of President Donald Trump last November.
"We are seeing a renewal of first principles, of people reasserting their right of self-determination against arrogant and paternalistic elites. As President Trump has said, ‘the forgotten men and women will be forgotten no longer,’" he said.
“Shaking up the status quo always takes time. We will have our share of speed bumps and roadblocks."
Mr Trump has said he would be willing to seek a trade agreement with an independent UK and discussed it with Theresa May, the Prime Minister, when she visited the White House in January.
A planned visit to the UK this summer by Mr Trump was postponed last month amid concerns over protests and snubs by MPs.
Mr Ryan offered his support to Mrs May over her snap election decision.
“Times like these call for bold leadership, and Theresa May is certainly setting the standard. Yesterday’s news and today’s vote is indeed a historic event," he said.
“And I know it will be a hard fought election, but hey, don’t expect too much sympathy from us... you see we in the House of Representatives are used to doing this every two years."