Ofcom said it has opened an investigation into Sky's "alleged abuse of a dominant position" regarding the wholesale supply of Sky Sports 1 and 2 - its flagship sports channels.
BT wants to offer Sky Sports via YouView set-top boxes to complement its own newly-launched sport offer.
But BT said Sky will not allow the channels to be broadcast over BT boxes unless it offers its rival wholesale access to its own BT Sport channels in return.
Ofcom said it will "consider whether Sky has abused a dominant position under UK and/or EU competition law".
The regulator added it will decide by next month whether to grant "interim relief" - which could temporarily give BT access to Sky Sports.