Unripe strawberry disaster!

Can anyone help with this problem.  We thought we had a beautiful crop of strawberries ripening nicely.  Today when I went to the allotment to water there are dozens of the unripe ones lying on the membrane.  They are covered in brownish blotches.  Not something I have ever seen before.  Fortunately they are not all affected but a large number.
Would be grateful for suggestons as to what it could be.  The adjoining bed is fine lovely ripe strawberries to pick!