Merill doctrine refers to a legal principle the government cannot be estopped from disavowing an agent's unauthorized act. Estoppel generally cannot be applied, at least offensively, against the government. Although originally based on principles of sovereign immunity, the doctrine also has been justified as deriving from separation of powers and public policy.
The standard was set in the case Federal Crop Ins. Corp. v. Merrill, 332 U.S. 380 (U.S. 1947). The doctrine is routinely applied to actions against federal agencies.