BRUSSELS, March 21 (Reuters) - U.S. gene sequencing company Illumina on Thursday won the backing of an adviser to Europe's top court who said EU antitrust regulators do not have the power to scrutinise its $7.1 billion bid for Grail nor block it.

The European Commission in 2021 asserted a rarely used power called Article 22 to assess the deal even though it fell short of the EU merger revenue threshold, following requests from several European Union countries.

It subsequently vetoed the deal and ordered Illumina to unwind the transaction which the company had completed before securing regulatory approval, leading Illumina to file a number of challenges on various grounds.

A lower tribunal in 2022 sided with the EU competition enforcer, prompting Illumina to take its case to the Luxembourg-based Court of Justice of the European Union (CJEU).

CJEU Advocate General Nicholas Emiliou said judges should set aside the General Court judgment because it had erred in its interpretation of Article 22 and that they should annul Commission decisions on referral request.

"In one fell swoop, the Commission would gain the power to review almost any concentration, occurring anywhere in the world, regardless of undertakings’ turnover and presence in the European Union and the value of the transaction, and at any moment in time, including well after the completion of the merger," Emiliou said.

Judges follow such non-binding opinions in the majority of cases.

The case is C-625/22 P - Grail v Commission and Illumina. (Reporting by Foo Yun Chee; Editing by Sudip Kar-Gupta and Mark Potter)