Enrichment values for ¹⁴C/¹²C and ¹³C/¹²C are reported for marine shells collected live and at known dates from the shoreline and coastal water of the United Kingdom. This data set indicates that an apparent age correction of 405 ± 40 years must be subtracted in the interpretation of conventional radiocarbon ages measured for samples of marine origin. Comparison of marine shell ages with the radiocarbon timescale established for the last glacial to postglacial transition infers that this correction factor can be applied over the past 12, 000 years. The need for apparent age adjustment must also be considered where human or animal remains may have been influenced by dietary intake of marine carbon. Ambiguities in the calculation and reporting of marine ages are discussed.