By Foo Yun Chee

BRUSSELS (Reuters) -Alphabet's Google faced a potential setback on Thursday as an adviser to Europe's highest court sided with EU antitrust regulators in the company's fight against a record 4.34 billion euro ($4.98 billion) fine levied seven years ago.

The European Commission in its 2018 decision said Google had used its Android mobile operating system to block rivals. A lower tribunal endorsed the EU finding in 2022 but trimmed the fine to 4.1 billion euros, prompting Google to appeal to Europe's top court.

Advocate-General at the Luxembourg-based Court of Justice of the European Union (CJEU) Juliane Kokott, in her non-binding opinion, advised the court to dismiss Google's appeal and confirm the reduced fine set by the lower tribunal.

"The legal arguments put forward by Google are ineffective," she said.

Kokott dismissed Google's argument that regulators should compare Google with a rival in assessing the situation.

"It is not realistic, in the present case, to compare the situation of Google with that of a hypothetical as-efficient competitor. Google held a dominant position in several markets of the Android-ecosystem and thus benefited from network effects that enabled it to ensure that users used Google Search," she said.

Judges, who usually follow four out of five such non-binding opinions, will rule in the coming months.

"Android has created more choice for everyone and supports thousands of successful businesses in Europe and around the world," a Google spokesperson said on Thursday.

"We are disappointed with the Opinion which, if it were followed by the Court, would discourage investment in open platforms and harm Android users, partners and app developers."

Regulators said Google's illegal practices dated back to 2011, as it required manufacturers to pre-install Google Search and its Chrome browser together with its Google Play app store on their Android devices. It paid them to pre-install only Google Search and blocked them from using rival Android systems.

Google's Android system, which it lets device makers use for free, runs about 73% of the world's smartphones, according to Statcounter.

The world's most popular internet search engine has racked up a total of 8.25 billion euros in fines linked to three investigations stretching back more than a decade, while other probes are ongoing.

The case is C-738/22 P Google and Alphabet v Commission.

($1 = 0.8726 euros)

(Reporting by Foo Yun Chee; Editing by Bernadette Baum)