When you are lucky enough to find a new job (or if you’re so unhappy in your current job you decide to leave anyway), you’ll need to formally tell your current employer you are leaving. Writing the letter can be tricky, but here are a few tips.
First of all, you should almost never be in a situation where the letter is the first your boss knows you are leaving. In all but the most extreme circumstances, you should tell your line manager in person first, preferably in a private meeting; the letter is merely a formality. Be wary of telling other staff about your departure until management knows and everything is in place.
The main function of the letter is to include a few key details, namely that you are formally giving notice and your intended departure date. Anything else you add is purely a matter of politeness. For example, unless you utterly despise your job and colleagues (and perhaps even if you do), it is courteous to thank your boss for the opportunity and the support they have given you.
You don’t have to give a reason for leaving, and short of a simple one-line explanation such as “I will be taking up a new post elsewhere”, there’s no real purpose in doing so. Whatever you write, do not include anything that is negative or critical of your current employer. Even if you never intend having anything to do with the company again, there’s no point souring them on you unnecessarily. (There are of course exceptions for some jobs: Leeds MP Colin Burgon took his resignation letter as an opportunity to criticise his party’s political stance, creating great press interest in his views.)
It’s also worth making clear in the letter that you will be cooperative and helpful in the transition, for example in explaining the role to your successor, or in gathering together passwords and documents that the company will need once you have left.