The tile fell from the roof of the three storey mall on its opening day.
Nobody was hurt and Johnson brushed off the incident while praising the construction team on the site.
But it has refocussed attention on the race to open the centre on time and two accidents last week when a shop fitter broke his back and a builder received an electric shock.
Jerry Swain, Regional Secretary of Ucatt’s’s London and South East Region, said: “The two recent accidents at the Westfield site are of extreme concern.
“Ucatt has been working diligently around the clock in a proactive manner in order to prevent accidents from occurring.
“These accidents underline just how dangerous the industry is, particularly when workers are required to work long hours in a rush to finish a project.
“The industry needs to review how safety is maintained in the final stages of projects so that accident risks are significantly reduced.”