London Mayor Boris Johnson said a Labour Government "is the greatest barrier to competitiveness" in the UK.
However, when he asked what the greatest threat was during his conference speech, a silence fell over the hall.
As someone shouted out that it was visas he exclaimed, "Not visas! Much worse than visas. What is the greatest threat we face? Come on folks - pay attention!"
Eventually a delegate got the correct answer and was met with applause and cheers.
More top news
Speaking publicly about their divorce for the first time, the actress said she is confident she and Brad Pitt will remain a family unit.
Misty low cloud and some patchy fog again tonight but with mild air
Former world title challenger Michael Watson and his carer were assaulted by would-be thieves in an apparent car-jacking attempt.