A federal judge has thrown out a redistricting plan in Cranston that put all of the inmates at the state prison into one city ward.

U.S. District Judge Ronald Lagueux  on Tuesday found Cranston's plan was "constitutionally untenable" and ordered the city to come up with a new redistricting plan in 30 days.

The lawsuit was brought by the Rhode Island chapter of the American Civil Liberties Union, which called the plan "prison gerrymandering."

Lagueux found that the plan, which put more than 3,000 inmates at the Adult Correctional Institutions into one of Cranston's six wards, had the effect of diluting the voting power of people in other wards.

He says the city must subtract the inmates from the city's total population in the new plan.