Prior to the Companies Act 1980 public companies, as such, were not defined but were a residual class, and only private companies were identified by definition. The position is now reversed.
The Companies Act 1980 defined a public company as a company limited either by shares or guarantee which has a share capital and which states in its memorandum of association that it is a public company. It must register or reregister under the Companics Act 1980 as a public company and its name must end with the words ‘public limited company’. These words can be abbreviated to P.L.C. If the memorandum states that the company’s registered office is in Wales then the Welsh equivalent of these words or abbreviations may be used. All public companies must have a nominal share capital of the ‘authorized minimum’ at present £50,000 – of which at least 25 per cent has been paid in cash plus any premium due. The minimum number of members is two. It must also deliver to the Registrar of Companies a declaration signed by a director or the secretary that the requirements j outlined have been complied with, together with a copy of the amended memorandum (where applicable) and an official application signed in the same way as the declaration. The Registrar will issue a certificate and only then may the company style itself a public company. Any company not satisfying these conditions will be a private company. This latter category will include guarantee companies without a share capital and also unlimited companies.
Reference: The Penguin Business Dictionary, 3rd edt.