The talk "Future perfect - sums for a sustainable society" would have discussed ways that engineers can address issues of sustainability, and had been arranged for 8 November. The talk was pulled at the eleventh hour.
RSA Wales director Carol Creasey said: "We were approached by the Structurals, who suggested we promote a joint lecture. I said I had seen Mark give an excellent speech here before, and why don't we have him again.
"Everything was fine until about two weeks before, when I had a phone call from the IStructE saying the event could not go ahead."
Whitby said he believed that the IStructE thought he would repeat his controversial lecture calling for the disbanding of institutions that fail to deliver for society.
"I remain bemused at how a talk set up months in advance could be cancelled by the head office of the Structures at the last moment without any reasons being given," he said.
"I can only assume that they didn't want me to repeat – not that I was planning to – the lecture I gave at the Structures headquarters
earlier in the year, where I discussed the duty that all institutions owe to society and how our separate identities, possibly, no longer serve the membership, let alone society at large."
IStructE chief executive Keith Eaton said Whitby was "100% incorrect" in his assertion.The RSA had now re-scheduled the event without the cooperation of the Structurals.