Latvala and team-mate Petter Solberg had a comfortable advantage on the leg's last two stages, the Fiestas proving better suited to the increasingly slippery conditions than their Citroen rivals.
That meant Latvala was able to establish a 12-second lead, as Solberg grabbed second place from Sebastien Loeb.
The world champion is now 9.2s off Solberg and 21s adrift of leader Latvala. Although Loeb insisted he was still pushing, he acknowledged that with a very comfortable championship lead, being beaten in Wales would not be a disaster.
Loeb has still managed to keep Mads Ostberg's Adapta Ford at arm's length. The fourth-placed Norwegian is now 11s behind Loeb.
Mikko Hirvonen continues to struggle for speed in the second factory Citroen and is almost a minute down on Latvala in fifth.
There has been nothing to choose between M-Sport Ford's two full-time drivers all day, with Ott Tanak staying just ahead of team-mate Evgeny Novikov in sixth despite power steering worries.
Thierry Neuville is running a steady eighth for Citroen Junior, while Matthew Wilson is ninth in his first World Rally Championship appearance since Monte Carlo in January.