I would like to resign my position as (insert job title) and intend to leave on (insert date).
Making this decision has been difficult as working at (Name of Employer) has been a positive experience. I have gained much here and have enjoyed working with you and my colleagues.
However, I am excited about my new position which I am undertaking, and my decision is irrecoverable as I am now legally bound by having signed a contract.
I am conscious of the need to provide support to the department until my leaving date and I shall give my full commitment as required.
I wish you and (Name of employer) every success in the future and thank you for the opportunities I have been given during my time here.