The Control of Dogs Order 1992 states that the owner of a dog or the person in charge of a dog that is not wearing a collar in a public place shall be guilty of an offence against the Animal Health Act 1981.

Dogs must wear a tag embossed with the Name, Address and Telephone Number of its owner.

Design your Dog Tag Set

The following was taken from the Government Web Site

1992 No. 901

CONTROL OF DOGS

The Control of Dogs Order 1992

Made 19th March 1992
Coming into force 1st April 1992

The Minister of Agriculture, Fisheries and Food, in relation to England, the Secretary of State for Scotland in relation to Scotland, and the Secretary of State for Wales in relation to Wales, in exercise of the powers conferred on them by sections 13(2) and (3) and 72 of the Animal Health Act 1981[1] and of all other powers enabling them in that behalf, hereby make the following Order:

Title and commencement
1. This Order may be cited as the Control of Dogs Order 1992 and shall come into force on 1st April 1992.
Wearing of collars by dogs
2.—(1) Subject to paragraph (2) below, every dog while in a highway or in a place of public resort shall wear a collar with the name and address of the owner inscribed on the collar or on a plate or badge attached to it.

(2) Paragraph (1) above shall not apply to—
(a) any pack of hounds,
(b) any dog while being used for sporting purposes,
(c) any dog while being used for the capture or destruction of vermin,
(d) any dog while being used for the driving or tending of cattle or sheep,
(e) any dog while being used on official duties by a member of Her Majesty's Armed Forces or Her Majesty's Customs and Excise or the police force for any area,
(f) any dog while being used in emergency rescue work, or
(g) any dog registered with the Guide Dogs for the Blind Association.

Offences
3. The owner of a dog or the person in charge of a dog who, without lawful authority or excuse, proof of which shall lie on him, causes or permits the dog to be in a highway or in a place of public resort not wearing a collar as prescribed in article 2(1) above shall be guilty of an offence against the Animal Health Act 1981.
Seizure of dogs
4. Any dog in respect of which an offence is being committed against this Order may be seized and treated as a stray dog under section 3 of the Dogs Act 1906[2] or under section 149 of the Environmental Protection Act 1990.
Enforcement
5.—(1) This Order shall be executed and enforced by the officers of a local authority (and not by the police force for any area).

(2) In this article "local authority" and "officer" have the same meaning as in section 149 of the Environmental Protection Act 1990.

Revocations
6. The Orders specified in the Schedule to this Order are hereby revoked to the extent specified in column 3 of that Schedule.

In witness whereof the Official Seal of the Minister of Agriculture, Fisheries and Food is hereunto affixed on

John Selwyn Gummer

Minister of Agriculture, Fisheries and Food

16th March 1992.

Allan Stewart

Parliamentary Under Secretary of State, Scottish Office

19th March 1992

David Hunt

Secretary of State for Wales

17th March 1992

http://www.opsi.gov.uk/SI/si1992/Uksi_19920901_en_1.htm