The United States is ready to strike a free trade deal with Britain as soon as possible, the Speaker of the House of Representatives said.
Paul Ryan, a senior member of Donald Trump's Republican party, told a central London audience that US lawmakers are committed to working with the president towards a bilateral agreement with the UK.
His clear signal provides a significant boost for Theresa May as European Union leaders take the first steps towards agreeing a negotiating position for forthcoming Brexit talks.
In a speech for the Policy Exchange think-tank, Mr Ryan said: "Now that Article 50 has been invoked, the UK and the EU will determine the best path forward over the course of these negotiations.
"We want the parties to come together and strike a lasting agreement, a strong UK-EU relationship is in all of our best interests.
"In that same vein, the United States will work closely with our EU friends and chart a path forward on TTIP (Transatlantic Trade and Investment Partnership) negotiations.
"At the same time we are committed to working with President Trump and your Government to achieve a bilateral trade agreement between the United States and Britain.
"This is one of the bipartisan messages I bring with me, I bring Democrats and Republicans here to this room to this country today, to say 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 two people."
Mr Ryan's comments come as the Prime Minister prepares to hold talks on Thursday with Antonio Tajani, the president of the European Parliament, which will ultimately have to approve any post-Brexit deal between the UK and EU.
Answering questions from an audience including International Trade Secretary Liam Fox and other Cabinet ministers, Mr Ryan said he was confident the UK and EU could agree an exit deal.
"This is your country, you make those decisions," he said.
"But I assume you will get an agreement, I can't imagine you won't, and I just want you to know that as your special ally, as your greatest partner, we're going to stand with you through all of this.
"We think it's in everyone's interests to have a strong relationship between Britain and the EU, we want to, as soon as the UK is ready and able, to do a trade agreement, a bilateral trade agreement."
Mr Ryan added: "I think what we're going through if we handle this correctly will be to bring more prosperity and security to our respective countries."