July 27 (Reuters) - U.S. natural gas company Spire Inc's Spire STL Pipeline LLC unit filed an emergency application with U.S. energy regulators to keep the STL gas pipe in service ahead of this winter to avoid gas service outages in St. Louis.

A spokesperson for Spire said on Tuesday that without STL in service "it’s estimated as many as 400,000 St. Louisans could be without natural gas service during peak cold conditions."

The filing follows a federal appeals court decision https://www.reuters.com/legal/litigation/fercs-rationale-pipeline-cert-weak-says-dc-circuit-2021-06-22 in June that threw out 2018 authorizations by the Federal Energy Regulatory Commission (FERC) allowing Spire STL to build a roughly $285 million gas pipe near St. Louis that is now operational.

The court decision was a victory against the regulator by the Environmental Defense Fund (EDF) which sued.

A unanimous panel of the U.S. Circuit Court of Appeals for the D.C. Circuit vacated a critical permits order for the 65-mile (105 kilometer) STL pipe after ruling that FERC adopted an "ostrich-like approach" when it found a market need for the line despite only one gas supplier, an affiliate of the line's operator, committing to use it.

Spire is the parent company of the line's operator Spire STL and of gas supplier Spire Missouri Inc, the affiliate that entered into a pre-construction deal committing it to use the line.

FERC authorized the pipeline in 2018. Construction began in 2019 after the commission held up EDF's challenge of the certification.

The line is designed to deliver up to 0.4 billion cubic feet per day of gas. It began operating in November 2019.

One billion cubic feet is enough gas to supply about 5 million U.S. homes for one day. (Reporting by Scott DiSavino; editing by Barbara Lewis)