(Undated) -- A new study suggests premium cable subscriptions are on the decline - and Netflix may be the reason. Research firm NPD Group found that more people are opting to pay for Internet video services instead of signing up to premium channels like HBO, Showtime and Starz.

The group's study shows subscriptions to those channels declined by six percent over an 18 month period between March 2012 and August 2013. During that same time, the study found a four percent rise in subscriptions to Netflix and other online video-on-demand services such as Hulu Plus and Amazon Prime Instant Video.

NPD's study does not provide conclusive proof that the availability of online services are the direct cause for the decline in premium channel subscriptions. But its analysts say they're comfortable in speculating that consumers are ditching premium TV for Internet-based substitutes.