NEW YORK, April 18 (Reuters) - Thornburg Mortgage Inc (TMA.N: Quote, Profile, Research) said on Friday that one of its units had failed to make a payment due on the final maturity date of $300 million in short-term notes, constituting a default under the unit's commercial paper program.

Thornburg, which earlier this month managed to avert a bankruptcy filing by raising $1.35 billion, said the unit was "bankruptcy remote" and that noteholders had no contractual recourse to Thornburg for nonpayment of the notes. (Reporting by Christian Plumb; editing by Carol Bishopric)