(Reuters) - NTT DoCoMo Inc on Monday said it has asked a London court to ensure Tata Sons Ltd finds a buyer for the Japanese carrier's stake in an Indian joint venture for $1 billion, after Tata failed to do so by an agreed date.

DoCoMo paid 266.7 billion yen ($2.22 billion) for 26.5 percent of Tata Teleservices Ltd in 2009. Under the agreement, holding company Tata Sons would sell the stake for at least half of the purchase price should undisclosed performance targets be missed, DoCoMo said in a statement.

DoCoMo requested the sale in July. That gave Tata Sons 90 business days to find a buyer for 72.5 billion rupees ($1.15 billion) or fair market price, whichever was higher, DoCoMo spokesman Shunsuke Muraki told Reuters.

As a buyer was not found by Dec. 3, DoCoMo filed an arbitration request on Jan. 3 with the London Court of International Arbitration to ensure the stake is sold, Muraki said.

"Worst case scenario, if the agreement is not followed through, a foreclosure is a possibility," Muraki told Reuters.

Tata Sons said in a statement on Monday it would continue with efforts to find a solution to the issue.

"From the outset, Tata Sons has been committed to honoring its obligations to DoCoMo, and has taken every possible step keeping in mind the interests of all stakeholders and in accordance with law," the company said.

"Tata Sons has made the necessary application to the Reserve Bank of India, and is awaiting a response," it said, without elaborating.