By Ian Walker 



  The U.K. Competition and Markets Authority said Monday that it is starting a formal review into Visa Inc.'s agreed acquisition of fintech startup Plaid Inc. to see if the merger would result in a "substantial lessening of competition" within the U.K. 


  The regulator has set a deadline of Aug. 24 to decide whether to allow the deal to proceed or to forward it for a deeper review. 


  Visa in January said it would buy Plaid for $5.3 billion, as part of an effort by the card giant to tap into consumers' growing use of financial-technology apps and noncard payments. 


  Write to Ian Walker at ian.walker@wsj.com