Labour has seized on an Evening Standard report which accused Boris Johnson of not registering shares in a television production company with the House of Commons authorities.
The party is seeking to portray the revelation as a blunder which undermines his promise to place City Hall’s register of interests on the internet.
Labour MP Karen Buck accused Mr Johnson of “trying to repackage himself as a serious and competent politician whilst claiming that his failure to comply with rules on reporting these share holdings to the Parliamentary commission was simply an oversight.”
Buck, who has written to Commissioner for Parliamentary standards over the matter, said “I am afraid this once again calls into question Boris Johnson’s competency for one of the most serious jobs in the country.”
Since the Standard’s report Mr Johnson has registered the shares in the MPs’ Register of Interests.