US President Donald Trump announced what he called a "breakthrough" trade deal with the United Kingdom on Thursday.

Trump said the "final details" of the agreement were still being "written up."

The modest deal leaves the 10% tariffs on UK imports, but slightly increases access to agricultural goods for both countries and lowers duties on UK car parts.

For its part, the UK reduced tariffs on US goods from an average of 5.1% to 1.8%.

A UK official told reporters that was "not a finished, classic 'bells and whistles' free trade agreement," cited by Reuters. He added that there was more serious work to do.

How did Trump describe the deal?

"The actual deal is a very conclusive one we think, just about everything has been approved," he said in the Oval Office.

Earlier Thursday, Trump posted to social media that the agreement would "cement the relationship between the United States and the United Kingdom for many years to come."

"It opens up a tremendous market for us," the US president said, acknowledging that he had not fully been aware of the restrictions American firms face to do business in the UK.

Trump said the deal affirms that "reciprocity and fairness is a vital principle of international trade," adding that it will increase access for US agricultural products.

The US president promised they would be finalized "in the coming weeks."

Starmer vows to defend UK workers and businesses

UK Prime Minister Kier Starmer on Thursday said, "This is a really fantastic, historic day," as he appeared in a joint video link with Trump during simultaneous news conferences in London and Washington.

Starmer said the deal "delivers on the promise to protect" UK carmakers and steel manufacturers.  

He had hinted at a forthcoming announcement earlier on Thursday.

"Talks with the US have been ongoing and you'll hear more from me about that later today," Starmer told a press conference in London. "But make no mistake, I will always act in our national interest, for workers, businesses and families, to deliver security and renewal for our country."

A British official suggested that the UK was considering lowering its own tariffs on US cars as well as cutting digital sales tax affecting US tech companies, Reuters reported.

Trump's tariff negotiations put to the test

Since taking office early this year, the Trump administration has been actively expanding the US tariff policies.

In April, Trump announced broad tariffs on US trading partners, however, most of the tariffs announced were later reversed or paused to allow for trade negotiations. However, the 145% levy on Chinese imports still stands.

The US president has repeatedly said that multiple countries are eager to finalize deals with the US.

Investors will be watching to see if Trump can deescalate the trade war he triggered, sparking chaos in stock markets around the world.

Officials from the US and China are meeting in Switzerland on Saturday to find a resolution to the trade war between the world's two largest economies.

UK eager for trade deals post-Brexit

The UK has sought to expand global trade ties ever since it left the European Union. But Brexit also brought some light relief when it came to dealing with Trump's tariffs — with the UK only suffering a 10% hit, half of the 20% announced against the EU.

Nevertheless, a trade deal with the US is seen as key by Starmer's government.

"The United States is an indispensable ally for both our economic and national security," a Downing Street spokesperson said. "Talks on a deal between our countries have been continuing at pace and the Prime Minister will update later today."

Britain earlier this week struck a free trade agreement with India — the biggest trade deal since Brexit.

On Tuesday, a UK official announced good progress amid talks with Washington on lowering tariffs on steel and autos. Discussions are also ongoing regarding Trump's recent announcement of 100% tariff on all movies produced outside the US.

Edited by: Alex Berry, Sean Sinico