WASHINGTON (Reuters) -The U.S. Supreme Court declined on Monday to hear a challenge by broker-dealer Alpine Securities claiming that the enforcement power given by the federal government to the Financial Industry Regulatory Authority, Wall Street's self-regulator, is unconstitutional.

The justices turned away Salt Lake City-based Alpine's appeal of a lower court's ruling that allowed FINRA to move forward with an enforcement action against the company for allegedly stealing more than $54.5 million from customers.

FINRA, a non-governmental self-regulatory organization, is responsible under federal law for supervising broker-dealers in the United States with the aim of protecting investors and the integrity of securities markets. FINRA has considered the expulsion of Alpine, accusing it of stealing from customers by charging excessive fees and misusing their investments.

Alpine responded with a lawsuit to block the FINRA expulsion proceeding, arguing that the organization's structure violates the U.S. Constitution. Specifically at issue is a constitutional principle called the private nondelegation doctrine, which involves limits on the ability of federal agencies set up by Congress to hand off authority to private entities like FINRA.

The U.S. Court of Appeals for the District of Columbia Circuit handed Alpine a partial victory in 2022, ruling that FINRA cannot expel member firms in expedited proceedings without obtaining review by the U.S. Securities and Exchange Commission, a federal regulatory agency.

The D.C. Circuit decided that a lack of SEC review likely violated the private nondelegation doctrine. But that court also let FINRA continue its enforcement proceeding against Alpine, saying it would not cause the kind of irreparable harm Alpine could face if later expelled, such as going out of business. This prompted Alpine's appeal to the Supreme Court.

FINRA opposed Alpine's appeal, as did President Donald Trump's administration.

Chief Justice John Roberts in March denied Alpine's emergency request to stop the FINRA proceeding.