BRUSSELS—British and European regulators threatened to crack down on Google and Facebook parent Meta over an agreement for online display advertising services, saying Friday that the deal may breach rules on fair competition. The fresh scrutiny in Europe, which has pioneering efforts to rein in big technology companies, strikes at the heart of Google’s business—the digital ads that generate nearly all of its revenue. In the “ad tech” marketplace bringing together Google and a constellation of online advertisers and publishers, the company controls access to the advertisers that put ads on its dominant search platform. Google also runs the auction process for advertisers to get ads onto a publisher’s site. The European Union’s top competition watchdog opened an antitrust investigation into a 2018 pact for Meta’s Audience Network to participate in Google’s Open Bidding program. The European Commission, the EU’s executive arm, said the deal, which Google internally dubbed “Jedi …