Reserve Bank of India, RBI, Governor Raghuram Rajan has said the willful defaulter tag is a powerful weapon in the hands of banks for resolving bad loans. His comments come days after the state-run United Bank of India declared industrialist Vijay Mallya as willful defaulter. He was speaking at a function in Boston, US. Mallya, whose Kingfisher Airlines owes money to several banks, had said he will challenge UBI's decision who declared him willful defaulter.Besides United Bank of India, Kingfisher owes about 9,140 crore rupees to about a dozen banks.