Product Owner introduces ungroomed (unfamiliar, not estimated) User Story(s) into the Sprint Planning meeting

Question

A problem that I am facing and would like some input into is; a Product Owner introduces ungroomed (unfamiliar, not estimated) User Story(s) into the Sprint Planning meeting.

The issue that this has caused is the team rushes to understand and estimate the User Story(s), which puts significant time pressure on the commitment portion of the Sprint Planning meeting. The team also seems to be unsure of their estimate due to the rushed nature of grooming them in the Sprint Planning meeting. The end result of this is a rushed, half hearted Sprint commitment, which is usually an under-commitment due to so much uncertainty.

I have seen two distinctly different causes for the late introduction of the User Story(s):

1. The team is new to Scrum and has been having difficulties grooming stories, prior to planning.
2. A brand new high priority User Story has appeared just before the Sprint Planning meeting.

I have discussed these issues with the Product Owner and we have decided upon on actions, I am wondering what have you tried when the Product Owner introduces a brand new high priority User Story just before or even in the Sprint Planning meeting? What worked, what failed?

The team that is having difficulty grooming User Stories in new to Scrum; hence I suspect that some facilitation of their grooming sessions, mentoring and some time will help them.

Do you have any other suggestions for helping teams come up prompt Planning Poker estimates?