An official publication is a publication published by Parliament, a government department (UK or foreign), devolved government or an international organisation such as the European Union or World Health Organization. Sometimes there is no personal author so the organisation is deemed to be the corporate author.
Department for Education (DfE, 2017) state that......
...(Department for Education (DfE), 2017).
DfE (2017) acknowledge that....
Note: A good tip if you are going to use the same citation a few times in your assignment is to use acronyms. You must explain the acronym in full the first time you use it, followed by the acronym so it is clear to your lecturer what the abbreviation stands for.
Group author. (Year). Title (Series or reference number). Publisher. URL
Department for Education. (2017). Unlocking talent, fulfilling potential: A plan for improving social mobility through education (Cm 9541). https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/667690/Social_Mobility_Action _Plan_-_for_printing.pdf
National Institute for Health and Care Excellence. (2020). Type 2 diabetes in adults: Management (NICE guideline NG28). https://www.nice.org.uk/guidance/ng28/resources/type-2-diabetes-in-adults-management-pdf-1837338615493
NB: When the author and publisher are the same (as in the example), omit the publisher name to avoid repetition.