A long form confidentiality agreement (also known as a non-disclosure agreement) for use in either:

A joint venture where both parties are disclosing information about businesses to be contributed to the joint venture.

A joint venture where one party is disclosing information about a business to be contributed to the joint venture and the other party is disclosing information about intellectual property rights to be licensed to the joint venture.

This document is in the form of an agreement rather than a letter and has been drafted for use in a UK context.