The 22 year-old had an eight-game loan spell at Banks's Stadium earlier this season, where he put in some impressive displays before being recalled to the City Ground due to an injury crisis at the club.
Darlow began his career with Aston Villa before joining the Nottingham Forest Academy aged 16, signing professional terms on 2010.
Karl has made two first-team appearances for the City Ground side and spent a spell on loan with Newport County.