BRUSSELS—U.S. life sciences company Illumina could face a hefty fine for completing its $8 billion cash-and-stock takeover of cancer detection test maker Grail without first securing EU antitrust approval. Illumina closed the Grail takeover on Wednesday and said it would hold the company separate while waiting for the European Commission to decide whether to clear or block the deal. But the EU executive said on Friday it would investigate if Illumina has breached its standstill obligation, which requires companies to secure EU antitrust approval before closing any merger deals. “We deeply regret Illumina’s decision to complete its acquisition of Grail while our investigation into the transaction is still ongoing,” Commission Vice President Margrethe Vestager said in a statement. “This obligation, that we call standstill obligation, is at the heart of our merger control system and we take its possible breaches very seriously,” Vestager said. Illumina on Friday referred Reuters to …