"Nobody wins from a trade war," British Prime Minister Keir Starmer said in an article for The Sunday Telegraph newspaper amid concerns over new U.S. tariffs.

"The economic consequences, here and across the globe, could be profound," he wrote in the opinion piece, adding that "old assumptions can no longer be taken for granted."

U.S. President Donald Trump has announced a new set of so-called "reciprocal tariffs" on imports from all trading partners, imposing a baseline 10 percent rate on goods from Britain.

U.S. and global stock markets tumbled on Thursday and Friday following new Trump tariffs, which include a 10-percent "minimum baseline tariff" on all imports starting April 5 and an "individualized reciprocal higher tariff" on the countries and regions with which the United States "has the largest trade deficits," set to take effect on April 9.

"Free trade is a galvanizing force for British export businesses," Starmer wrote in his article, noting that turning away from free and open trade "would be a grave mistake."

He stressed the importance of keeping calm and fighting for the "best deal," adding that he would "only strike a deal if it is right for British business and the security of working people."

In the article, he also called for "strengthening alliances and reducing barriers to trade with other economies around the world," saying that "agile diplomacy delivers a stronger, more diverse, and secure economy" for the country.

On Saturday, hundreds of thousands of protesters gathered in dozens of cities across the United States and Europe to protest the controversial policies of Trump's administration.