The ten metre high structure was being used to construct a 60 tonne wall on a section of the route near Dyce.

One worker told the Aberdeen Press and Journal: “The cage was large and it could have done damage, it happened around 4.30pm, if it had been another time I’m sure there could have been fatalities.”

Transport Scotland confirmed it is investigating the incident which occurred last week.

UCATT officials claim they were not informed of the collapse.

Regional Secretary, Steve Dillon, said: “We want answers to not only why our members’ lives were put in jeopardy but why our convenor was kept completely in the dark.”

A Transport Scotland spokesman told the Journal: “We can confirm that a health and safety incident occurred on site last week.

“No one was injured as a result of this incident. An investigation is currently under way and any lessons will be applied to future work.

“Transport Scotland takes health and safety very seriously and is working with the contractor to ensure the highest health and safety standards are maintained across the site.”

The AWPR is being built by a consortium of Balfour Beatty, Galliford Try and Carillion.