By Tom Westbrook, Ankur Banerjee, Amanda Cooper and Shivansh Tiwary

(Reuters) -Shares of global automakers took a dive on Thursday after President Donald Trump put a wall of tariffs around the U.S. automotive sector, exacerbating worries over the impact on global trade and potential hits to industry profits.

Trump on Wednesday followed through on weeks of threats for new tariffs on imported cars, saying a 25% import tax on vehicles not built in the U.S. would begin on April 3.

Although the duties have been well flagged, shares in automakers across the globe tumbled on Thursday, with U.S. auto giant General Motors sliding 7% in premarket trading and Ford Motor shedding about 4%.

Trump's tariffs leave American automakers vulnerable after years of building an extensive cross-border supply chain and expanding assembly facilities in Mexico and Canada that are crucial to supporting their U.S. sales.

As European markets opened, shares in Volkswagen, Europe's top car maker, dropped 2%, while those in luxury brands BMW and Mercedes-Benz fell about 3% each.

In Japan overnight, some $16.5 billion was wiped off transport stocks, according to LSEG data, as shares in Toyota fell 2.7%, Honda 3% and Nissan 2.2%. Hyundai Motor and Kia in South Korea dropped about 4% each.

"Mexico, Japan, South Korea, Canada and Germany are the biggest suppliers of auto-related products to the U.S. and stand to lose out if Trump doesn't back down," said Russ Mould, investment director at AJ Bell.

"It's another blow to relations between the U.S. and the rest of the world, and a further reason for investors to be gloomy."

Volkswagen is in the frame since 43% of its U.S. sales are sourced from Mexico, S&P Global Mobility estimates, as is Chrysler-owner Stellantis, which, along with Ford, is one of the top producers of U.S. vehicles based in Mexico.

The head of Germany's car industry association said the tariffs are a "fatal signal" for global trade.

"The risk of a global trade conflict - with negative consequences for the global economy and growth, prosperity, jobs and consumer prices - is high on all sides," VDA President Hildegard Mueller said in a statement, calling for bilateral U.S.-EU talks to find a solution.

The U.S. administration had set a deadline of April 2 to unveil its broader policy on tariffs, meaning the hit to shares was less dramatic than when Trump first threatened non-U.S. manufacturers with extra charges.

But the signal - hurting allies and car buyers - was nevertheless unsettling for markets which have been slow to accept that the levies may become permanent fixtures and drive lasting changes in world trade flows.