An expression introduced in a 1984 newspaper article by David Rosewarne claiming to describe “a newly observed variety of English pronunciation” that was a “mixture of non-regional and local south-eastern English pronunciation and intonation”. It has been much discussed in the popular press, but early accounts were impressionistic, contained unsubstantiated or erroneous claims and tended to include features such as choice of vocabulary and of syntactic constructions, which of course have nothing to do with pronunciation.
A serious controlled study of the supposed new variety was published by Przedlacka in 2002. A summary can be found here. Further information about the press reaction to Estuary English, and more measured accounts of its phonetic and phonological features can be found here.
Przedlacka, J. (2002) Estuary English? Frankfurt: Peter Lang.