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Over the last sixty years, the development of synthetic and durable materials, namely plastic, coupled with a growing human population, has resulted in a rapid increase in the levels of anthropogenic debris in rivers, along coastlines and in the wider marine ecosystem. Currently, an estimated 4.8 to 12.7 million tons of plastic enter the oceans every year but this is expected to increase to between 9.6 and 25.4 million tons by 2025. As such, it is one of the most widely recognised pollution issues facing the planet due to its wide-ranging ecological and socio-economic implications. The main aims of this thesis were to i) examine citizen-science beach clean data to better understand the composition of anthropogenic litter deposited on British beaches by determining the most common items, materials, sources and pathways, and exploring the data for spatial patterns and temporal trends in litter density; ii) investigate an indirect pathway (trophic transfer) of microplastic (< 5mm in size) ingestion in marine top predators by analysing scat (faeces) from captive grey seals (Halichoerus grypus) and the wild-caught fish they were fed upon; iii) explore the extent to which wild marine mammals ingest microplastics and consider the potential implications by examining the digestive tracts of 50 marine mammals from 10 species that stranded around the British coast; iv) develop a method of investigating dietary exposure of marine mammal top predators to microplastics, by combining scat-based molecular techniques (metabarcoding) with a microplastic isolation method. The research carried out for this thesis reveals that i) plastic is the main constituent of marine litter on British beaches and the majority of traceable items originate from land-based activities, such as public littering. The coasts of the southwest England and south Wales have the highest litter levels and certain items - small plastic fragments, plastic food packaging, wet wipes, polystyrene foam, balloons and large fishing net - are increasing; ii) trophic transfer is an indirect and under-studied, but potentially major, route of microplastic ingestion for marine top predators; iii) microplastics are ubiquitous within the digestive tracts of wild marine mammals stranded around the British coast but the overall low abundance suggests they may be egested; iv) the rate of microplastic ingestion by marine top predators may be related to the type of prey they consume but further work is needed to assess the impacts of this omnipresent pollutant.