Preempt legal action by disclosing that you're accepting a job at a competitor with this resignation sample letter.
April 15, 2012
Dear Mr./Ms. Bossname,
This is a difficult letter for me to write but I feel that as a direct, responsible boss you deserve total honesty. I'm afraid I must respectfully resign my position here at (company,) effective (date.) I am happy to stay on to train my replacement, and will ensure that I have completed all unfinished projects as well.
In the interest of full disclosure, I should let you know that I have accepted another position at (competitor name.) You should know that I did not seek out this position, but rather was contacted by management and offered the job. While I have learned a lot about (field) and made many contributions I am proud of here, I feel that my education and skills will be better served at my new place of employment.
I thank you in advance for your understanding and apologize for any inconvenience this causes you. I wish you and the company the best of luck in the future.