When did Royal Mail change to large letter format?

Royal Mail introduced the large letter format in August 2006 as part of a restructuring of the cost of its postal services. The new system, known as pricing in proportion, takes into account the size and thickness of items in addition to their weight.

Items posted with a large letter stamp should weigh no more than 750 grams and be no thicker than 25 millimeters. The maximum permitted length of a large letter is 353 millimeters and the maximum width is 250 millimeters. Brochures, single DVDs in standard boxes and slim magazines can all be posted using large letter stamps. Larger items are classed as parcels and incur higher postage fees.