A letter containing a settlement offer and written "without prejudice save as to costs", that is with the express reservation of the right to refer the letter to the court on the question of costs if the offer is not accepted. Use of Calderbank letters has reduced significantly since the advent of Part 36 offers as they do not automatically have the same costs consequences. However, there are still some circumstances in which Calderbank letters remain a useful mechanism.

For more information, see Practice note, Calderbank offers and Standard documents, Calderbank letter from solicitors acting for a defendant (with drafting notes) and Without prejudice save as to costs settlement offer letter from a claimant (with drafting notes).