The ltd.uk and plc.uk domains are intended to allow a unique mapping between company names registered under the Companies Act with Companies House and a DNS name. Although it was envisaged by the Naming Committee that only ltd.uk was required, Nominet UK's legal advisors have said that a public limited company registering within ltd.uk would be committing a criminal offense. Therefore the additional plc.uk is required.

Section 33 of the Companies Act 1985 states: .
1. A person who is not a public company is guilty of an offence if he carries on any trade, profession or business under a name which includes, as its last part, the words "public limited company" or their equivalent in Welsh ("cwmni cyfyngedig cyhoeddus").

2. A public company is guilty of an offence if, in circumstances in which the fact that it is a public company is likely to be material to any person, it uses a name which may reasonably be expected to give the impression that it is a private company.

Note: Unlimited companies registered under the Companies Act, unincorporated bodies, partnerships and bodies incorporated under other legislation are not eligible for an entry in ltd.uk or plc.uk.

The only characters from a registered company name allowed in a DNS name are the letters A to Z, a to z, the numbers 0 to 9 and hyphen. These will be known as the "allowable" set. Certain words will be removed from the registered company name where they occur at the end of the name before the DNS name is generated, these are known as the "reserved" words.

They are "limited", "public limited company" and the Welsh Language versions ("cyfyngedig" and "cwmni cyfyngedig cyhoeddus") along with the abbreviations of these words, e.g. ltd, plc, cyf and ccc.

The rules are:

One name per company name registered at Companies House

The name shall correspond exactly to the registered name save for characters not in the "allowable" set which shall be interpreted as below and the "reserved" words. Where the definite article appears as the first word of the name it may be omitted

White space between words shall either be ignored or replaced by a single hyphen "-" (see 3 below for how the interpretation is made)

All other characters shall be removed

Company names written in a non English character set are not allowed in this version. Note:

It is believed that these rules represent the way that Companies House interpret names in order to produce a unique match. In the event that they do not and/or that Companies House rules change in the future, the first successful applicant for a name will be assigned the name.

In the event of a dispute over a name an appeal shall be allowed to the Council of Management of Nominet UK. The grounds for the appeal shall be in writing and the decision of the Council of Management shall be final.