As of the 23rd March 2010, tyres with a width of 185mm or lower can no longer be sold by retailers unless they have a 'S' marking on the sidewall, please note that this 'S' will be at the end of a different set of numbers than the ones representing your tyre size (see diagram for details).
This 'S' shows that the tyre complies fully with a recent piece of legislation (called the 2001/43/EC Directive, or 'S Marking Regulation') created by the European Union to minimise the amount of noise produced by tyres when they are in contact with the road.
This now means that the next time you buy a new set of tyres, and the tyres you require have a width of 185mm or lower, then whoever you purchase them from, must supply tyres with a 'S' marking on the sidewall.
Over the next 18 months this rule will extend to other tyre sizes:
From October 2010, tyres with a width up to and including 215mm must comply
From October 2011, all tyres will have to comply.
So, that's the facts, but we're sure that you are more concerned as to how this affects you and your future tyre purchases.