WASHINGTON (AFP) - The United States said on Friday (Dec 13) it was ready to seek a free-trade deal with Britain once it leaves the European Union, an exit made certain by Prime Minister Boris Johnson's sweeping election win.

Johnson's Conservative Party campaigned on a pledge to strike an accord with Washington, offering an economic fillip despite the loss of the vast common European market closer to home.

"The United States is committed to the US-UK shared global agenda, including expanding our robust economic relationship by reaching a comprehensive free-trade agreement with the UK once it formally withdraws from the European Union," State Department spokeswoman Morgan Ortagus said in a congratulatory message.

President Donald Trump's administration has repeatedly voiced enthusiasm for a free trade deal with Britain, supporting Brexit as an assertion of sovereignty.

But any agreement could face hurdles in Congress where lawmakers want assurances that Brexit and its re-erection of borders will not jeopardize the fragile peace in Northern Ireland.

Trump - who, like Johnson, has emphasised nationalism and periodically stunned the public with outspoken remarks - earlier on Friday hailed the British prime minister's win as a harbinger for his own chances next year.

Trump's approach contrasts sharply with his predecessor Barack Obama, who warned before the 2016 Brexit vote that Britain would be at the "back of the queue" if it left the European Union.

An aide to Obama later said his public warning - using the British "queue" rather than the more American "line" - came at the request of the then prime minister, David Cameron, who campaigned against Brexit in the referendum he called himself.