A new paid app for the Xinhua Dictionary - regarded as the highest authority on the Chinese language - has been criticized for charging for the product.

Commercial Press, which publishes the book, launched the app for Android and Apple devices on Sunday, and almost instantly came under fire when users discovered that only their first two searches were free.

After that, they must pay 40 yuan ($5.90) to continue using the service - more than double the price of a hard copy sold online.

"Why should we pay when there are so many ways to look up a word, such as on Baidu?" asked one netizen on Sina Weibo. "It only makes sense if there is no alternative to your product."

Jiang Yan, operations director for Shanghai Shangdi Digital Publishing Technology, which developed the app, said on Tuesday that the charge will remain.

"The app is being download more than 100,000 times a day, far more than we expected," he said. "The cost of research and development, operation, maintenance and follow-up services should all be included in the pricing."

He added that the app is not in the same category as the print edition, since users can keep the app without any worries about it becoming damaged or outdated, and it will be updated regularly.

Xinhua Dictionary was the first to use pinyin, a system for transliterating Chinese characters into a Latin alphabet. Its first edition came out in 1953 and 567 million hard copies had been sold as of July 2015.

Generations of Chinese used the dictionary as a reference book to learn Chinese characters in primary school. Last year, Guinness World Records recognized it as the "most popular dictionary" and the "best-selling regularly updated book".

The mobile app offers a link to a snapshot of the printed page, and has Li Ruiying, a former anchor for China Central TV, pronounce each character. It also illustrates a word's stroke sequence.

Hu Xiaoming, a professor of Chinese language and literature at East China Normal University, said he supported a "reasonable and necessary charge" for the mobile service.

"It's a matter of intellectual property. Unlike some online sharing platforms like Wikipedia, Xinhua Dictionary is more authoritative," he said.

Cao Chen in Shanghai contributed to this story.