The U.K. and EU are planning separate systems to tax carbon-heavy imports, prompting familiar fears about the impact in Northern Ireland.

In 2019, Boris Johnson promised his Brexit deal would do away with checks on the Northern Irish border. Five years later, the region is about to be whacked with yet another trade headache.

The latest tension comes from net zero rules, with the U.K. and EU preparing separate schemes on either side of the border to tax climate-damaging imports.

Britain’s proposals on high-carbon imports don’t kick in for at least a year after the EU’s start — leaving Northern Ireland, and its half-in half-out status, at risk of getting caught in the middle of yet another Brexit-driven political row.

Experts say the latest dispute will hike business costs and threaten jobs in the region, and could reopen arguments about laws separating Northern Ireland from the rest of the U.K.