A cycle lane marked by a solid (unbroken) white line. It is 'mandatory' in the sense that drivers of motor vehicles are not permitted to enter it, rather than being mandatory for cyclists to use it. Drivers are not permitted to cross the unbroken white line.
Somewhat confusingly, as the law stands, it is no longer an offence to park in a mandatory cycle lane if that lane was created after 2016 (although it is an offence to park in a mandatory cycle lane if it was created before 2016). For these reasons mandatory cycle lanes are, for the moment, best employed with double yellow lines, which explicitly prohibit parking.
Prior to 2016, mandatory cycle lanes required a Traffic Order to implement. This requirement was removed in the 2016 TSRGD.