A £37m schools reorganisation in the south Wales valleys has been quashed after a judicial review.
Several schools in Pontypridd would have been shut and replaced with two "super schools" for children.
Sixth forms would also have closed and concentrated at one school and a further education college.
But a High Court judge ruled that Rhondda Cynon Taf council had failed to take into account the impact of changes on Welsh medium education.
RCT leader Andrew Morgan said the council's overriding objective had been to "positively transform the delivery of education, through the mediums of both Welsh and English".
Earlier this month the council's cabinet voted to press ahead with the plans, despite 435 objections.
Council leaders had said the reorganisation would improve the quality of education but protesting parents took them to judicial review.
In his judgement, High Court judge Mr Justice Fraser said the council had broken rules by not referring the closure of a sixth form to the Welsh Government and had failed to take into account the impact on the future of Welsh language education.
"The fewer pupils who enjoy a Welsh medium primary education, the fewer are likely to attend Welsh medium secondary education," the judgement said.
"Such pupils are "lost for ever", he added.
Lawyers for the campaigners argued that the Welsh language version of the relevant laws were clearer than the English language version about the requirements and responsibilities of the council.Barrister Rhodri Williams QC said: "This is indeed a landmark ruling for the significance of Welsh legislation in Wales.
"Never again will it be sufficient to argue that the English language version of a statute dictates what the meaning of the law is.
"From now on, all those concerned with the proper implementation of legislation in Wales will need to bear in mind both language versions.
"At long last, true equality before the law for Wales's two languages has been established by the courts."
Cathy Lisles, of campaign group Our Children First (Ein Plant Yn Gyntaf), said they were delighted the reorganisation had been squashed.
"This is not just a victory for concerned parents and pupils in Pontypridd but for all in Wales.
"We hope and trust that rather than continue the legal fight, the council will reach out to us and work constructively going forward to improve education for all in RCT."
'An important issue'
Welsh Language Commissioner Aled Roberts, said the case was an "important issue for the future of Welsh-medium education"
"This case makes it clear to councils that in all cases, pupils must be able to continue receiving education with at least equivalent standards and opportunities in their chosen language," he said.
"This is not an option, but mandatory in Wales."It also states clearly that, with any proposals that are going to have an effect on Welsh-medium education, a thorough impact assessment must be carried out. This is not optional either."
The council said it would need to consider the content of the judgment fully, before determining how to respond to the decision.
Mr Morgan said: "Throughout this process the overriding objective of the Council has been to positively transform the delivery of education, through the mediums of both Welsh and English in the Greater Pontypridd area, by delivering significant investment, which would improve the opportunities available to young people."