BRUSSELS, Aug 20 (Reuters) - U.S. life sciences company Illumina (ILMN.O) could face a hefty fine for completing its $8 billion cash-and-stock takeover of cancer detection test maker Grail (GRAL.O) 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. read more 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 the statement it issued on Wednesday that said re-acquiring Grail and keeping it separate was the only way to meet the deadline for completing the deal. Violations of EU rules can lead to fines of as much as 10% of the aggregate turnover of the companies. French telecoms group Altice for instance was hit with a 125 million euro ($146 million) fine three years ago for closing its 2015 takeover of PT Portugal before gaining regulatory approval. EU antitrust regulators had warned on July 22 that Illumina"s bid could curb innovation and competition as they opened a full-scale investigation into the matter. read more Illumina"s decision to close the Grail deal came ahead of an Aug. 24 trial by the U.S. Federal Trade Commission, which has also voiced concerns about the impact of the deal. Grail makes a non-invasive, early detection biopsy test to screen for many kinds of cancers using DNA sequencing.