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."