A state court ruled Friday that prisoners must be counted among voters back in their home neighborhoods rather than in upstate prisons for the purpose of redrawing state legislative districts, a likely blow to the slim Republican majority in New York’s Senate.

Although prisoners cannot vote, the decision means more voters will be counted as living in heavily Democratic New York City and other urban areas as part of the redistricting process, which is tied to the census. That would reduce the population upstate and likely result in fewer seats in the Assembly and Senate representing sparsely populated upstate areas where prisons are located.

The Senate’s Republican majority says it will appeal the ruling by a trial level judge in Albany.

The immediate practical result of the decision could be minor. The state redistricting commission is already redrawing legislative districts by following a 2010 law requiring prisoners to be counted in their latest home neighborhoods.