I would like to quit a job I recently took up for another better offer that has come along my way. It has been just a week since I joined this firm, but I have indulged in no legal/contractual bonding so far with them.

I now look forward to politely inform them that I no longer would like to continue working there through a formal mail response. It would be great if someone could suggest me the format of documenting the same without appearing too mean or unethical.