A contract termination letter is a formal notification to the other party that you will be terminating the contract.
If the letter is being sent as a hard copy, then it should be in the proper business format, otherwise, if the letter is being emailed, it should have an appropriate subject line and proper grammar.
As of [Contract End Date], this letter states that we will no longer need your services. Thank you for all of your work and time. This relationship has supported us for [Length of service]. At this time, we have decided to move in a different direction because of financial and business reasons.
Please note that this letter is our official notice that we do not wish to renew or renegotiate our contract with your group. This letter is dated outside the 30 days notice required in the contract. If you have any questions, feel free to contact us. Thanks again for all of your time and work. We truly appreciate it.