^^ They were. The first evidence is in TOS' 1st season. You don't have to look any further than "The Menagerie." What possible point could there be in chasing after the Enterprise warping out of orbit if your vehicle isn't warp capable?
Anecdotal evidence throughout the series supports the very same conclusion. And nothing in the series says one way or the other, but how the shuttlecraft are used argues that they are indeed warp capable.
Anecdotal evidence throughout the series supports the very same conclusion. And nothing in the series says one way or the other, but how the shuttlecraft are used argues that they are indeed warp capable.