A federal agency opened a new avenue for Republicans to loosen regulations governing banks and Wall Street.

In a letter dated Tuesday, the Government Accountability Office found that the Federal Reserve and other regulators had issued guidance to influence the behavior of banks in a way that is subject to congressional oversight. At least in theory, the agency’s decision will make it easier for Congress to relax regulations that were imposed after the 2008 financial crisis.

The G.A.O.’s letter followed a request by five Republican lawmakers, led by Senator Thom Tillis of North Carolina, to examine the way the Fed had pushed banks to alter their conduct. The agency found that some of what regulators called “guidance” actually amounted to official rules, which are subject to the Congressional Review Act. That law lets Congress overturn rules issued by federal agencies.

The G.A.O. opinion is the latest victory in a broad campaign by Republicans and the Trump administration to ease constraints on banks. With a new law and a series of changes to capital requirements and trading restrictions, they have already made substantial progress.