Apple's lucrative deal with Google could be under threat after a U.S. judge ruled that the Alphabet-owned search giant was operating an illegal monopoly.

A potential remedy for Google to avoid antitrust actions could involve terminating the agreement, which makes its search engine a default on Apple devices, Wall Street analysts said on Tuesday.

Google pays Apple $20 billion annually, or about 36% of what it earns from search advertising made through the Safari browser, for the privilege, according to Morgan Stanley analysts.