A US judge has ruled Google acted illegally to crush its competition and maintain a monopoly on online search and related advertising.

The landmark decision on Monday is a major blow to Alphabet, Google’s parent company, and could reshape how technology giants do business.

Google was sued by the US Department of Justice in 2020 over its control of about 90% of the online search market. It is one of several lawsuits that have been filed against the big tech companies as US antitrust authorities attempt to strengthen competition in the industry.

This case has at times been described as posing an existential threat to Google and its owner given its dominance of the search and online advertising business. It is unclear yet what penalties Google and Alphabet will face as a result of the decision. The fines or other remedies will be decided in a future hearing.

The government has asked for “structural relief” - which could, in theory at least, mean the break-up of the company.

In his decision, US District Judge Amit Mehta said Google had paid billions to ensure it is the default search engine on smartphones and browsers.

“Google is a monopolist, and it has acted as one to maintain its monopoly,” Judge Mehta wrote in his 277-page opinion. Alphabet said it plans to appeal against the ruling.