How to write a letter declining a supervisor offer? I have written a proposal to a supervisor for asking a letter of support in order to apply for a PhD research grant. First, I emailed to supervisor A but no respond for two weeks. Then, I tried to write to the second supervisor (let say supervisor B) and he agreed to write me a letter of support. A week later, I got an offer from supervisor A. I decided to put both letters in my application by list supervisor B as prior. 

Recently, I have been awarded a grant under a supervisor B. So, I would like to write a letter to say thank and sorry politely to supervisor A. 

Please advise how to write a fit letter of this situation.
A few suggestions:


Don't mention the order of events or the fact that you listed B as the prior, unless A asks. That is, something like "I had applied for this grant listing several potential supervisors, as both yourself and B are able to supervise this work. etc. etc." ; if A asks why you didn't list him/her as the prior, then mention the timing.
If there is the theoretical possibility of you asking for a change of sueprvisor for the grant from B to A, you will (sort of) need to justify why it is you're not doing so.
If co-supervision is an option, you'll need to either ask for from A, or explain why it's not relevant.
I would try to organize some sort of visit with A, or suggest other ways you might collaborate or that you might be useful to him (although there might not actually be such an option, in which case forget about it).