Liam Fox has told MPs it was a mistake to allow the "distinctions to be blurred" between his personal relationship with close friend Adam Werritty and his professional life.
The defence secretary admitted Werritty was with him in a "social capacity" on 18 foreign trips and met him at the Ministry of Defence 22 times in 16 months.
Fox had been under intense pressure in the last few days after it emerged Werritty, who despite having no official government job or security clearance, accompanied Fox on foreign trips, set up meetings for him and described himself as his adviser.
He told the House of Commons on Monday that Werritty "at no point" had access to classified documents or had classified security briefings.
But shadow defence secretary Jim Murphy said Fox had offered only a "partial apology".
"We may never know what got the secretary of state into this crisis whether it was arrogance naivety or hubris," he said.