Alphabet Inc's Google on Monday sought to rebut criticism that it enforces its 30% tax on sales of paid apps arbitrarily, saying the just over 3% of paid apps that are out of compliance must follow the rules within a year.

The announcement follows lawsuits last month by "Fortnite" video game maker Epic Games accusing Google and Apple Inc of anticompetitive conduct by requiring that their payment systems be used in apps, so that they can collect a portion of sales. The companies are defending the allegations.

App stores are a fast-growing business for Google and Apple, as their main business - search ads and iPhone sales, respectively - flatten out.

Google said under 3% of developers with apps on its Play store sold digital goods over the last 12 months, and nearly 97% comply with its payment system policy.