I am writing to confirm the discussion which we had on [date].
As you are aware, your employment with the company was subject to a probationary period. Following regular reviews, your performance, unfortunately, has not reached a satisfactory standard during your probationary period. Your probationary period ends on [date] and it has been decided to terminate your employment at this point.
Before leaving the organisation you must ensure that you return all company property [e.g books, laptop, phone, security pass].
It is an unfortunate outcome, but I do wish you every success in the future.