By Krystal Hu

Feb 1 (Reuters) - ChatGPT, the popular chatbot from OpenAI, is estimated to have reached 100 million monthly active users in January, just two months after launch, making it the fastest-growing consumer application in history, according to a UBS study on Wednesday.

The report, citing data from analytics firm Similar Web, said about 13 million unique visitors used ChatGPT per day in January, more than double the levels in December.

“In 20 years following the internet space, we cannot recall a faster ramp in a consumer internet app," UBS analysts wrote in the note.

Similar Web did not immediately respond to a request to confirm the figures. OpenAI, a private company backed by Microsoft Corp, launched ChatGPT in late November.

It took TikTok about nine months after its global launch to add 100 million users and it took Instagram 2-1/2 years, according to data from Sensor Tower.

ChatGPT is currently free to use. The company on Thursday launched a $20 a month subscription that provides more stable and faster service as well as the opportunity to try new features first.

Analysts believe the viral launch of ChatGPT will give OpenAI a first-mover advantage against other AI companies. The growing usage, while adding up to a substantial amount of computing cost, has also provided valuable feedback to help train the chatbot's responses.

The tool has also raised questions about academic dishonesty and misinformation.

Last month, Microsoft announced another multi-billion investment in OpenAI in the form of cash and cloud credit. (Reporting by Krystal Hu in Toronto; Editing by Cynthia Osterman)