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Donald Trump’s new tariff plans risk getting bogged down in protracted legal challenges as the president relies on obscure laws to wage his trade war after the top court in the US ruled many of his previous duties illegal.

The Supreme Court struck a blow to Trump’s signature economic policy last month when it ruled the president could not use emergency powers to hit countries with big duties. 

The Trump administration immediately reimposed tariffs of 10 per cent on US trading partners by invoking a different piece of legislation, Section 122 of the Trade Act of 1974.

Trump now faces another court battle over those tariffs, with hearing dates set for next month for two lawsuits already filed against the levies at the Court of International Trade.

Section 122 allows the president to impose tariffs of up to 15 per cent for 150 days to address “large and serious balance-of-payments deficits”.

Lawyers in one of the cases argue that Trump “has once again exercised tariff authority that he does not have — involving a statute that does not authorise the tariffs he has imposed — to upend the constitutional order and bring chaos to the global economy”.

Legal experts say the administration — which plans to defend the use of Section 122 — could find the court will allow Trump broad leeway to use tariffs for reasons of economic security.

Kathleen Claussen, a professor of law at Georgetown University, said the courts had “historically, in the last 10 years . . . deferred always to the president on these economic security issues”.

However, even if the fresh tariffs survive the immediate legal challenge, Trump’s reliance on a patchwork of laws is likely to slow down his trade war and leave him with less flexibility to impose tariffs at the rates and timing that he wants.

There are signs that Trump has already been held back by the rules governing how tariffs can be applied under Section 122.

Trump initially threatened to impose tariffs of 15 per cent on countries following the government’s loss in the Supreme Court case, but the threatened rate triggered a backlash among European allies that struck deals for preferential lower tariff rates last year.

Both the UK, which agreed a blanket tariff of 10 per cent with the US, and the EU would be worse off under a blanket 15 per cent tariff on top of existing tariffs than under the deals they made.

Lawyers say Trump may be unable to set different rates for different countries under the law now being used.

“The statute is quite clear that it should be applied on a non-discriminatory basis, one rate across all countries, with some exceptions for particular products,” said Peter Harrell, who was a senior economic official under the Biden administration.

“I think that if he moves the rate around, and certainly if he has differential rates . . . the court would then probably rule against these tariffs. They’re inviting lots of additional legal problems.”

Trump is now relying on his US trade representative, Jamieson Greer, to complete several new investigations into trading partners as quickly as possible to grant him the right to apply future higher tariffs to countries under Section 301 of the Trade Act of 1974.

Earlier this month, the USTR launched probes into 60 trading partners over their practices around forced labour, and more into whether American trading partners had “excess capacity and production in manufacturing sectors”.

Greer told reporters this month that he hoped to hand the president those options before the Section 122 tariffs expired.

Everett Eissenstat, a former Trump trade official now at Squire Patton Boggs, said Section 301 investigations normally take six to eight months to complete.

“This would be fast,” said Eissenstat — although he added that the USTR might be able to make Trump’s trade deals with other countries “pretty durable” by relying on this process.

Trump may also be able to extend his current tariffs to buy his administration more time, although doing so would probably invite further legal challenge.

Although the text of Section 122 says the tariffs can only be extended beyond 150 days with approval from Congress, Claussen said Trump could exploit a legal grey area to attempt to roll his levies out further.

“It can’t be that once the president uses this once, that it can never occur again. So what’s the space . . . you have to wait between this one and the next one?” Claussen said.

“The statute doesn’t tell us. Obviously this violates the spirit of the statute. But I just remind everybody that we’re not in a spiritual world.”

The White House did not respond to a request for comment.