Washington DC - US President Donald Trump unveiled a "breakthrough" trade agreement with Britain on Thursday, celebrating the first deal in a tariff war that has rattled the world.

Speaking by telephone from the Oval Office to British Prime Minister Keir Starmer, Trump said the deal would be the first of many with other countries.

The deal will reduce tariffs on British cars and steel, but a 10% baseline levy on British goods will remain.

Britain will in return open up markets to US beef and farm products despite consumer concerns over their quality.

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"I'm thrilled to announce that we have reached a breakthrough trade deal with the United Kingdom," Trump said at the White House.

"The deal includes billions of dollars of increased market access for American exports, especially in agriculture."

Starmer said it was a "really fantastic, historic day," noting that it coincided with the 80th anniversary of Victory Day for allied forces – including Britain and the US – over Nazi Germany in World War II.

The trade deal slashes export tariffs for British cars from 27.5% to 10%, Britain said.

"That is a huge and important reduction," PM Starmer said during a visit to a Jaguar Land Rover factory in the central Midlands area of England.

The deal was focused on specific sectors rather than being a fully-fledged free trade agreement sought by the UK following its 2020 departure from the European Union, the official said on condition of anonymity.

As well as dealing with steel and car tariffs, it would lay out positions for discussions over possible future tariffs in the key pharmaceuticals sector.

Both sides said there would be further negotiations on a fuller deal, but Trump denied overselling the agreement.

"This is a maxed-out deal – not like you said it really incorrectly," he added, answering a reporter's question on whether he was overstating the breadth of the deal.

The deal follows a charm offensive by Starmer, who came to the White House in February armed with an invitation from King Charles III for a historic second state visit for Trump.