(Reuters) - Congress can block President Barack Obama from using federal immigration fees to issue permits for millions of undocumented immigrants to stay and work in the United States, according to a congressional research memo released on Wednesday.

The memo from the non-partisan Congressional Research Service to Republican Senator Jeff Sessions could provide a boost to him and other conservatives who are pushing for a December budget confrontation with Obama to try to stop his executive order overhauling the U.S. immigration system.

A plan to deny such funds in a must-pass federal spending bill next month, as part of a Republican effort to fight Obama's immigration move, could heighten the risk of another government shutdown.

House Appropriations Committee Chairman Hal Rogers, a more moderate Republican, has argued against such a strategy, pushing instead for a spending bill that fully funds government agencies through September 2015. He has said that simply "defunding" immigration agencies would have little effect because the primary agency that would process the applications could still operate on the fees it collects.

But the Congressional Research Service said lawmakers could halt operations of the U.S. Citizenship and Immigration Services agency by including language explicitly prohibiting the use of funds by a specific agency for a specific purpose.

"If a statute were enacted which prohibited appropriated funds from being used for some specified purposes, then the relevant funds would be unavailable to be obligated or expended for those purposes," the research service said in the memo.

It went on to say that such legislative language would also apply to funds collected through fees but noted that courts could have different interpretations.