Former S4C chief executive Arwel Ellis Owen has said the Welsh language TV channel is in "grave danger" of becoming entirely reliant on the BBC licence fee.
Ellis Owen explained he believes the UK government could announce an end to its £6.7m funding from 2015 in its spending review on Wednesday.
"I believe there is a grave danger that the government will take £6.7m away from S4C's budget on Wednesday, which will break the chain between the money from the government and S4C, which will leave S4C completely reliant on the licence fee from 2015."
S4C Authority chair Huw Jones has also expressed "huge concerns" about the channel which received £6.7m from the Department of Culture, Media and Sport (DCMS) in 2013, down from £101m in 2010.
The BBC will contribute £76.3m for the licence fee in 2013-14, and £74.5m in 2016-17. The independence of the broadcaster was reported as being safeguarded in an agreement outlining its relationship with the BBC earlier this year.
Speaking on BBC Radio Wales' Sunday Supplement programme that being "at the mercy of" [the BBC] was "a very relevant issue in this whole debate".
He added: "That is why it's important, not only for the financial reasons, for S4C to have a continued funding from an alternative source, but also in terms of its independence it's very importance as well."
The Welsh TV producers' trade body TAC has sent an open letter to UK culture secretary Maria Miller calling for a halt to cuts to S4C funding.
Its open letter read: "If every £1 you invest in S4C generates nearly £2 for the Welsh economy, it follows therefore that conversely, for every £1 of S4C's budget that is cut, the Welsh economy will lose double this amount."