Can, Could, May

Permission

We use can, could and may to ask for permission. We use can and may, but not could, to give permission. May is less common:

Compare

asking for permission	formal/polite?	giving  permission	formal/polite?
Can I ask you a question?	informal	Yes, you can. Yes, you may. Yes, you could.	informal
Could I use your phone?	more        formal/polite		formal/polite
May I use your phone?	even more  formal/polite

http://dictionary.cambridge.org/us/grammar/british-grammar/can-could-or-may