The total number of mobile subscribers of the company fell to 6.16 mln users in Goa, Mumbai and Maharashtra by the end of November.
A month earlier, the number was 6.47 mln and two months earlier, it was 6.98 mln.
On Oct 12, Bharti Airtel said it had agreed to take over the mobile business of Tata Teleservices, along with some liabilities.
In the same month, Tata DoCoMo saw a steep drop of 7.3% in its mobile subscriber base. In the previous month of September, when there were rumors of a shut down of the service, the company had seen a 3.5% decline.
The numbers suggest that if the regulators take much longer to the approve the merger between Tata Telservices and Bharti Airtel, the company could see a substantial loss of subscribers by the time it merges with Bharti.
Tata DoCoMo has already started dismantling its network.
To avoid disruption to its subscribers, it has entered into an intra-circle roaming agreement or ICR with Bharti Airtel.
It has also announced new plans, including an unlimited calling plan at Rs 148.