Edwardian Corset Cover sometimes called an under bodice. I think this was all part of a trousseau set made by Hilda Thompson as it does bear a label with her name and I would think dates to early 1900s.  It is made from a very fine cotton and has a lovely pleat detail at the back.  As on other pieces from this trousseau  collectionon the bodice is beautifully embroidered and has lace insets and edging to the arms and neck.  It is fastened with tiny mother of pearl buttons and has three little pin tucks either side at the waist opening, Some of the lace is a little ragged at the edges and there is a tiny catch on the back but given the age it is in remarkably good condition.
Dimensions these are all approximate:
Waist 70cm
Bust 96cm which equates to an approx. 36-38 bust size
Shoulder to bottom of flounce: 51cm
Armhole shoulder to side seam :18cm