The news comes in the wake of the broadcaster's interim results, which saw profits fall from £97m last time to £65.8m.
The broadcaster's long-term rating has been downgraded from A- to BBB+ and its short-term rating from A2 to A1. The outlook on both ratings remains negative, the agency said.
"The downgrade reflects Carlton's weakened financial profile resulting from the substantial costs of developing ITV Digital and related digital programme channels," Standard & Poor's said in a statement.
Last month Carlton, which owns ONdigital -- soon to be rebranded as ITV Digital -- with ITV partner Granada, reported that the digital service was unlikely to break even until 2004. ITV Digital has 1.1m subscribers and a high churn rate of 22% compared with Sky Digital's 5.4m customers and churn rate of 10%.
"Ratings could be lowered further in the medium term if the commercial success of ITV Digital is delayed and Carlton's credit ratios do not recover sufficiently," Standard & Poor's added.
A ratings degrade can result in higher interest rates on borrowing for companies.
Carlton's shares climbed 1p to 393p at 2.57pm in London.