According to Stamp Magazine:
1. Self-adhesive stamps are to be printed with a security inscription on the entire backing paper from next year to deter forgers. This will include special stamps as well as definitives in counter sheets, booklets, business sheets and coils.'Next year' of course means 2015 and sure enough we now know that printing of the 2nd class Business Sheet on 1 December (01/12/15 - and of course there may have been earlier printings) has the new backing paper, which we are referring to as SBP - Security Backing Paper.
Now, the question is how to collect these - if indeed you want to add them to your collection. The stamps themselves are not different, they are coded M15L MBIL just the same as those already in circulation.
The inter-stamp gaps on Business Sheets are not very large, so you may want to collect in pairs, or a strip of 3 with one stamp removed. With Booklets of 6 or 12, the spacing shows more backing paper on the 1st and 3rd (and 4th and 6th) columns, but the 2nd and 5th columns are as close as the Business Sheets, so again, maybe strips of 3. For Counter Sheets pairs, or marginal singles (especially top and bottom) will show more of the backing paper than single stamps.
So, what do you think, how will you want to collect - let me know, as dealers shouldn't dictate by limiting what is available. And do you think this will be any deterrent to forgery?
As with the new coil, we now have these in stock.
UPDATE: Very interesting comment by stamp printing expert Glenn Morgan in the comments, which I urge all readers to read in it's entirety.
This feature is not there for Joe Public to spot, but it is an essential part of security when bringing a case to court by the very investigators that you write about, who have Royal Mail support by its introduction of such features.
See the Tullis Russell website for a picture of the Canadian stamp with printing on the reverse. Again, impossible to see on a stamp which is being used, but for major stocks of suspected forged stamps, then the absence of printing on the reverse is evidence.