The Dangerous Substances in Harbour Areas Regulations 1987

2.  Each hazard warning sign shall be in the form of a square set with its sides at an angle of 45° to the vertical and the length of the sides shall be–

(a) in the case of signs on hazard warning panels, not less than 200 millimetres;

(b) in the case of signs on the labels for compartmented tank barges, not less than 95 millimetres;

(c) in the case of signs, other than those for substances in Class 7, to be affixed to a freight container, portable tank or receptacle, not less than 100 millimetres, except that, in the case of receptacles that are of such dimensions that they can only bear smaller signs, the sign should be as large as is reasonably practicable;

(d) in the case of signs for substances in Class 7 to be affixed to a freight container, portable tank or receptacle, 100 millimetres.