A dispute between the U.K. and European Union over coronavirus vaccines has deepened, as London said it would have to slow down its inoculation program because of a cut in supplies.

The country’s National Health Service blamed the looming short-term reduction of doses on a drop in "inbound vaccines,” adding to tension with Brussels over shipments. That was after the EU threatened to block exports to its former member state, prompting British Foreign Secretary Dominic Raab to suggest the bloc was engaging in brinkmanship and that "the world was watching.”

The further souring of relations comes as the EU gets increasingly desperate to accelerate the pace of vaccinations, while Britain wants to ensure its inoculation program remains a stand-out success after recording the continent’s highest death toll from COVID-19.