Newel post removal

[ibo63]

Hi All, I want to remove the lower portion of my newel post which runs from the landing through the floor and should be resting on the ground floor. I say 'should be' because it isn't at the moment. The ancient concrete and quarry tile floor has been removed and the section of it under the newel post has also come away - it wasn't my intention but it happened. I'm creating an office in this area and a stud wall will be in place beneath the stairs in the sameposition as the plywood partition in the pictures. Clearly the newel post is supporting the top three stairs as they turn and also the string runs into the post. Is it possible to cut the newel post off just below the tread which is painted white? I don't believe the newel post has been supporting very much for the last several years as it is rotten at the base anyway. It may already be or could be bolted to the first floor beam running next to it, or might it be possible to run a beam across beneath any cut into the wall on either side (span is only 1.6m). Any ideas would be gratefully received. I was intending it to stay in position but it's removal would free up a lot of space. Thanks in advance.