UK optical discs manufacturer, located in Blackburn, and operating under the legal name of PDO Discs Ltd. from December 1986 until March 1996.
(Some items released in late 1996 are known to still display a Made in U.K. by PDO credit, possibly due to the use of old stock by the successor company, PMDC.)

Regularly credited in CD matrices with (capitalized) "Made in U.K." or "Made in U.K. by PDO" (or P.D.O.), PDO Discs Ltd. was the UK branch of PDO (Philips DuPont Optical) based in Blackburn, Lancashire.

Originally established in 1980 by Philips Electronics to manufacture Laser Discs, by December 1986 the plant also manufactured CDs, CDRs, and CDVideos. In April 1996 control was handed over to PMDC, UK after the plant was sold to PolyGram.

PDO UK had a strictly sequential numbering system, unique to this plant:
Between 1987 and June 1989 the pattern consisted of 6+2 digits:
XYYYYZ NN (e.g. 134231 01)

where X = 1 (first disc) or 2 (second disc for 2xCD editions) and Z = number of discs in the set
YYYY = sequential manufacturing number,
NN = 01 up to 09 or even higher (potentially the glass master number).

From July 1989 onwards, the system was enhanced to 8+2 digits, with optional symbol presumably identifying the LBR:
XYYYYYYZ NN (e.g. 10133491 01 or 20133491 01 %)

where X = 1 (first disc) or 2 (second disc for 2xCD editions),
YYYYYY = sequential manufacturing number,
NN = 01 to 09, 10, 11, etc. (potentially the glass master number).

The pattern was valid for all discs types (CD, CD-Single, CD-Mini, or CDV).

Only add XYYYYZ and XYYYYYYZ as LCCN catalog number.
For 2xCD editions add Made By - PDO, UK twice with one number each.

Note: The pattern can also be found on other UK plants such as Mayking or VDC. In such cases, PDO should be added for Glass Mastered At plus sequential number.
See an example here.

Due to manufacturing problems at the UK PDO plant, many releases from the late 80s and early 90s eventually suffered from CD bronzing (or "disc rot"), and are rendered unplayable. When the problem became known in the early 1990s, PDO (and later its successors) offered to replace any discs thus affected, and pledged to re-press new CDs to meet the demand.
However, the helpline was discontinued in 2006, and defective CDs are now no longer replaced, even though some of the affected record labels continue to offer replacements.
Such repressings can be identified by a matrix number starting with 198xxxxx, where the 198 usually replaces the first 1 to 3 numbers of the original number (see this example, where 198 replaced the 1).

The Blackburn factory changed hands several more times, being operated by Deluxe (7) and eventually being acquired by EDC and operating as EDC Blackburn Ltd until its closure on December 31, 2009 with all operations moving to EDC's main European plant in Hanover, Germany.