Can or may?

Taken from The New Oxford Dictionary of English:

Is there any difference between may and can when used to request or express permission, as in may/can I ask you a few questions?

It is still widely held that using can for permission is somehow incorrect, and that it should be reserved for expressions denoting capability, as in can you swim? Although this use of can is not regarded as incorrect in standard English, there is a clear difference in formality between the two verbs: may is, generally speaking, a more polite way of asking for something and is the better choice in more formal contexts.