The UK cannot begin negotiating with the US or any other country until its obligations as a member of the EU end, Chancellor Philip Hammond has admitted.
Theresa May is expected to trigger the Brexit process in March and the disentanglement from the continent could take two years or more.
Mr Hammond tweeted this morning from the European Council's economic and financial affairs council: "At #ECOFIN with EU finance mins to deliver a clear message: UK will be an engaged EU member until we leave & close partner in the future".
He told reporters: "We will continue to abide by the rules, and the regulations and the laws, of the European Union for so long as we are members.
"Of course we want to strengthen our trade ties with the very many trade partners we have around the world, but we're very mindful of our obligations under the treaty and we will follow them precisely."
Ms May told Republicans on Thursday was was "delighted that the new administration has made a trade agreement between our countries one of its earliest priorities".
The Prime Minister said she was "looking forward to pursuing talks" about a new UK and US free trade agreement.
“It will take detailed work,” she told leaders of the GOP in a speech in Philadelphia. “But we welcome your openness to these discussions and hope that we can make progress so the new global Britain that emerges after Brexit is even better equipped to take its place confidently in the world.”
Ms May is only allowed to talk about deals with new partners in principle and cannot make any concrete attempts to negotiate free trade agreements before Britain leaves the EU.
She had earlier said: "We're at the start of the process of talking about a trade deal."
Meanwhile, Mr Trump told reporters: "I'm meeting with the Prime Minister tomorrow, as you know, of Great Britain. I'm meeting her tomorrow—I don't have my commerce secretary [and] they want to talk trade. So I'll have to handle it myself, which is OK."Reuse content