Can PO delegate the writing of user stories ?

The Scrum Guide says:

The Product Owner is also accountable for effective Product Backlog management, which includes:

• Developing and explicitly communicating the Product Goal;

• Creating and clearly communicating Product Backlog items;

• Ordering Product Backlog items; and,

• Ensuring that the Product Backlog is transparent, visible and understood.

The Product Owner may do the above work or may delegate the responsibility to others. Regardless, the Product Owner remains accountable.

User stories are an example of how Product Backlog items might be expressed, and which would be subject to further ongoing Product Backlog refinement.

If the creation of such items was left until Sprint Planning, you might wonder how the work could possibly be ready for consideration in that event.

My question is can he/she delegate the user story writing to the team which could then be further reviewed & refined during Backlog Refinement or "even during Sprint Planning (if required)" by the entire team.

Absolutely just as Ian has said and as per the Scrum Guide, the Product Owner can delegate "responsibilities" to the Developers and the Scrum Master however, he is still accountable for the PBL.

"even during Sprint Planning (if required)"

By the time the Sprint Planning is initiated the PBL would have been ordered for the Developers to select the PBI for the Sprint. Product Backlog grooming/refinement is the act of breaking down and further defining Product Backlog items into smaller, more precise items, this is not done during the Sprint Planning (SP). This is an ongoing activity of the PO outside the dedicated SP. 

@Shailiesh by the way, Ian is well experienced, a great coach, you have to get used to his coaching technique if you are new on this forum.