It was revealed that the Scot will step down from his role as manager of the Red Devils at the end of the current campaign.
Having won 49 trophies during his 27-year tenure at Old Trafford, Cameron even joked that he hopes Ferguson's departure would "make life a little easier" for his team Aston Villa.
"Sir Alex Ferguson's achievement at #MUFC has been exceptional. Hopefully his retirement will make life a little easier for my team #AVFC," the PM wrote on Twitter.
You can follow live reaction to Ferguson's retirement here.