Barrell claimed that the tree, planted at the Dorset estate by the Duke of Wellington in 1827, "was among the top 10, if not top five, heritage trees in the country" and "had the potential to be retained for many decades, if not centuries".
The trust commissioned a report by the Symbiosis tree consultancy, which described the tree as "in good overall physiological condition for a specimen of its size and age".
The report's author Mick Boddy concluded: "The tree is not as extensively decayed as the tomogram implies and, given its landscape value and historical significance, would suggest that removing the tree may be premature."
Barrell, who visited the site both before and after felling, said his photograph of the felled trunk "confirms that Symbiosis was absolutely right that the tree did not need to be felled".
He added: "There were clearly problems with this tree but those could have been easily dealt with by pruning. There is an outer ring of solid wood that is much more than 50 per cent of the trunk needed to be confident it was stable."
In response, the National Trust said its national garden historian, forestry head, the Garden History Society chief conservation officer and two external arboriculturists were involved or sympathised with the decision.
"As well as considering the obvious safety concerns, we also take into account context and location. Major pruning was an option offered by one external expert, perhaps delaying felling by several years, but cedars do not respond well to such treatment.
"This approach would also have meant the loss of this majestic tree's beauty and grandeur, and being located in Kingston Lacy's formal garden this aesthetic consideration was important in our final decision. The condition of the timber when the tree was finally felled supported our decision."