All UK employers will eventually be required to automatically enrol their eligible job holders into a qualifying pension scheme.
The date by which this must be completed depends on the size of your PAYE scheme on 1 April 2012.
The first enrolment date affected employers with 250 or more eligible job holders. Medium sized businesses (those with 50-249 eligible job holders) will require to auto-enrol those eligible job holders during the period 1 April 2014 to 1 April 2015.
The Pensions Regulator has provided a useful tool on their website. By inputting your PAYE reference the tool will provide the exact date by which you will require to auto-enrol your eligible job holders. You can find the tool here.