Congress may use its “power of the purse” to block President Obama’s executive action on illegal immigrants, a prominent legislative and policy authority on Capitol Hill concludes.

The new report from the Congressional Research Service says that the House Appropriations Committee was incorrect last week in saying the government agency most responsible for implementing Obama’s action—  U.S. Citizenship and Immigration Services — cannot be defunded by Congress.

The House spending committee reasoned that Citizenship and Immigration Services operates on revenue it generates through immigration applications, so it doesn’t use congressionally appropriated funds to pay for activities such as issuing legalization status and work permits.

>>> Republicans Differ Over ‘Power of Purse’ to Stop Obama Immigration Action

But the Congressional Research Service concludes that Congress still must appropriate the funds received from fees if the money is to be spent lawfully.

The Daily Signal obtained a copy of the report, prepared for Sen. Jeff Sessions, R-Ala., ranking minority member of the Senate Budget Committee. It says:

Amounts received as fees by federal agencies must still be appropriated by Congress to that agency in order to be available for obligation or expenditure by the agency. In some cases, this appropriation is provided through the annual appropriations process. In other instances, it is an appropriation that has been enacted independently of the annual appropriations process (such as a permanent appropriation in an authorizing act). In either case, the funds available to the agency through fee collections would be subject to the same potential restrictions imposed by Congress on the use of its appropriations as any other type of appropriated funds.

Still, CRS warns that lawmakers must be precise in the legislative language they use if they decide to prohibit agency spending to carry out the president’s changes to the immigration system.

The report concludes:

The context in which a restriction on funds collected through fees or otherwise made available is enacted will determine which agencies or activities will be subject to that restriction. It is at least a theoretical possibility that a court could find that such a restriction reached activities or agencies that were entirely fee-funded.

Sessions has vowed to use the budget process to fight Obama’s moves to provide legal status for up to 5 million immigrants here illegally, allowing them to stay and obtain work permits.

The current short-term spending measure funding the government expires Dec. 11.

This report has been modified.