Article 123(2) of the European Patent Convention states, "A European patent application or a European patent may not be amended in such a way that it contains subject-matter which extends beyond the content of the application as filed". The same rule also applies to UK patents under section 76 of the Patents Act 1977 (implemented in a "cack-handed way", according to Jacob LJ here). The basic rule for both is that no amendments can be validly made which would allow something to be claimed (or even described) that was not clearly also there when the application was filed.
The above issue, among several others, arose recently in the case of Teva v Merck, a judgment of Mr Justice Floyd from last week and available from BAILII here. Teva applied for revocation of Merck's EP(UK) patent 0509752, relating to "Ophthalmic compositions comprising combinations of a carbonic anhydrase inhibitor and a beta-adrenergic antagonist", an eye drop formulation for the treatment of glaucoma. After the usual extensive analysis, Floyd J found that the patent was invalid for being obvious over an earlier scientific article in light of the skilled person's common general knowledge (see PatLit here for more).
What the IPKat found more interesting, however, was the argument over added matter relating to one of the claims of the patent. The claim as granted covered a method for making the eye drop formulation, and included a final step of "adjusting the pH of the composition obtained to 5.0-6.0 by the addition of a suitable reagant". This feature was not in the claims as-filed but was added during prosecution, imported from one of 33 examples in the specification, which were whittled down to only 10 in the granted version. Although several of the examples had the step of adjusting pH, none had the range 5-6 together with the composition being claimed. As Floyd J put it, "Once the claim is limited to the specific co-formulation of dorzolamide and timolol, there is no basis for the disclosure of a process for making a co-formulation by adjusting to a pH other than 5.5 to 6.0" (para 70).