In both cases, the conclusions reversed the approach adopted by the Obama administration, which had suggested that people who found work through apps were likely to be considered employees. In 2016, the labor board issued a complaint against Postmates, an app-based delivery service, over allegations that the company had interfered with workers’ ability to exercise their labor rights.

The agency could not have issued that complaint without first concluding that Postmates couriers were employees.

The memo released on Tuesday, which was dated April 16, has no long-term value as a precedent and can be reversed by a future general counsel. But it carries considerable weight in how the board enforces federal labor law.

The general counsel is the labor board’s chief prosecutor and has authority over whether or not to issue formal complaints against employers, which are analogous to indictments in criminal law. The memo essentially tells Uber drivers and many other gig-economy workers that they should not bother reporting a labor rights abuse to the board because Mr. Robb has deemed them to be outside its jurisdiction.

Mr. Robb has had a long career in labor law largely spent representing employers. He was involved in the Reagan administration’s legal fight with the air traffic controllers’ union, which went on strike illegally in 1981. Labor experts have said the government’s decision to fire the controllers contributed to organized labor’s decline.

The immediate consequence of the memo is to render moot three formal accusations, filed in different parts of the country, that Uber had violated federal labor law. The memo instructs the board’s regional offices to dismiss the charges if the people who made them do not withdraw them first.

In its analysis, the general counsel’s office listed 10 factors that collectively determine whether a worker is an employee or a contractor, including the extent to which the company can control how the work is performed and whether the company or the worker provides equipment.