BRUSSELS — The European Union today slapped a $475 million fine on U.S. biotech giant Illumina for buying out cancer-screening company Grail without the approval of the 27-nation bloc’s antitrust watchdog.

Illumina had announced an $7.1 billion acquisition of Grail in 2020 but the European Commission, the EU’s executive arm, said it broke the bloc’s merger rules by moving ahead to complete the deal without its consent. Last year, the EU announced it was blocking the merger on competition grounds.

“If companies merge before our clearance, they breach our rules. Illumina and Grail knowingly and deliberately did so by implementing their tie-up as we were still investigating,” said EU antitrust Commissioner Margrethe Vestager. “This is a very serious infringement.”

The commission said companies almost invariably play by the rules and wait to complete an acquisition or merger until antitrust authorities have cleared it.

Illumina is a major supplier of next-generation sequencing systems for genetic and genomic analysis, while Grail is a health company developing blood tests to try to catch cancer early.

The European Commission argued at the time that the buyout would enable Illumina to squeeze out competitors and inhibit fair competition by acquiring too dominant a position in the market.

Only last month, Illumina CEO and director, Francis deSouza, resigned after the company’s chairman had been voted out by shareholders. It follows a monthslong heated battle with activist investor Carl Icahn, including the difficulties involving the Grail acquistion.