This is a stunning vintage dress by John Marks London. It is in a ribbed white cotton blend fabric, with contrasting turquoise striped fabric. It has a sweetheart style neckline with a self tie strap, and a pretty tiered long skirt. The bodice is lined, not the skirt. It fastens with a back zip. It's vintage and there are some very small marks of discolouration, but they are hard to see and not obvious, as the photos show. 

It is made in England and labelled as a size 12 - but as it is vintage, it will likely be smaller than a modern size 12. (Fits a modern size 8-10)

Measurements are 
Armpit to armpit 16" (there is some stretch to this - can go up to 17")
Waist 14"
Length 49.5"