The council's cabinet had recommended changing Brecon High School to an English-medium school from 1 September.
But 32 members of full council opposed the plans, with eight in favour.
The vote was taken as an indication of the full council's views but it was not a final decision. That rests with the cabinet which meets on 14 March.
A council report said low pupil numbers and the "need to ensure robust linguistic progression and appropriate curriculum choice for Welsh-medium pupils" were the reasons for the recommendation.
The council plans to centralise Welsh-medium education for mid and south Powys in Builth Wells High School, which is a 40 mile round trip.
A consultation has been carried out on the plans, with staff, parents and pupils raising a number of concerns.
Among them were pupils missing out on extra-curricular activities, such as after school clubs and music lessons, due to having to travel to Builth Wells for Welsh-medium education.
A majority of councillors voted in favour of retaining a Welsh-medium stream at a Powys school on Tuesday.