Super Furry Animals musician Cian Ciaran and Hinterland actress Mali Harries are among a cast of famous names calling on Cardiff Council to only open Welsh medium schools in the future.
They say current lack of provision means children from poorer backgrounds are not getting the chance to access to Welsh language education.
More than 35 other Welsh personalities have signed the letter - including Eisteddfod-crowned poet Catrin Dafydd, Cardiff-born musician Heather Jones and Pobol y Cwm star Siw Hughes.
The group says the Welsh capital should be blazing a trail when it comes to Wales' mother tongue - especially given the Welsh Government's target of a million speakers by 2050 .
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The letter says: "More parents, from every background, wish to see their children grow up to be adults that are able to speak Welsh. However, at present, Welsh medium provision in Cardiff is totally insufficient.
"Families across the city are being turned away from Welsh medium education because of oversubscribed schools.
"Children from some of the most deprived backgrounds are being prevented from access to language and all the opportunities that it offers, due to a lack of information and a lack of provision. We are calling on you to transform this situation and to ensure that the Welsh language is accessible to everyone in the city, from St Mellons to Ely."
Mabli Siriol, of Welsh language campaign group Cymdeithas yr Iaith Gymraeg, added: “Unfortunately, at present, the increase in Welsh medium provision is not sufficient enough to meet the great demand for places in Welsh medium schools, nor for Cardiff to reach its local targets on the journey towards 2050."
Cardiff Council said it had worked hard on making Welsh more accessible to school children.
Deputy leader Sarah Merry, said: " We currently have enough capacity in our Welsh-medium secondary schools, and there are still places available.
"In the short to medium-term, the predicted increased demand can be met by increasing places using the capacity within our existing three Welsh-medium secondary schools.
"Our Welsh-medium capacity will continue to increase to reflect overall demand."
Councillor Merry added that increasing places too quickly could put existing schools "at financial risk".
Since 2012, the council has opened a number a number of Welsh schools including Ysgol Bro Edern and Ysgol Treganna as well as expanding places at others - such as Ysgol Melin Gruffydd.
A spokesman said it has also secured, in principle, £284m to double the size of Ysgol Nant Caerau and Ysgol Pen y Pil.
