MIAMI — The Florida Supreme Court on Thursday rejected political gerrymandering by state legislators and ordered eight congressional districts redrawn within 100 days, a decision likely to complicate preparations for next year’s elections.

In the 5-to-2 decision, the justices concurred with a trial court’s finding that a 2012 redistricting map drawn by the Republican-led Legislature had been tainted by “unconstitutional intent to favor the Republican Party and incumbent lawmakers,” and that Republican “operatives” and political consultants “did in fact conspire to manipulate and influence the redistricting process.”

“This was a complete victory,” said David King, the lead lawyer for a group of plaintiffs, including the Florida League of Women Voters and Florida Common Cause, who had challenged the new map. In a conference call with reporters, Mr. King said the Supreme Court had evidently had no trouble finding a pattern of “partisan conduct on the part of both the Legislature and the partisan political operatives” who drew the boundaries.

The justices said the legislators had violated anti-gerrymandering measures approved by Florida voters in 2010. The court also chastised the legislators for conducting much of their redistricting business in private, and for destroying pertinent documents and emails even after vowing to be transparent about the process.