European Commissioner Jonathan Hill said that Boris Johnson's comparison of the EU with the aims of Hitler could affect potential Brexit negotiations.
Johnson sparked a backlash last week after he told the Telegraph that the Nazi leader had failed at unifying Europe under a "single authority" and the EU was now trying "to do this by different methods".
Speaking to ITV News' Robert Peston, Lord Hill said that if the Leave campaign wins the June 23 referendum, negotiations with the EU could be impacted by his comparison.
Johnson's comments were described as "divisive" and "hysterical" but supporters said they were "historical analysis".
Lord Hill said that his comments were unlikely to "warm people up for a negotiation".