Work on the 80,000-seater stadium in Stratford was due to begin in August but the date was moved forward.
The Olympic Delivery Authority said they have been able to make an early start because of progress already made on site.
After the Games, the £500 million stadium will be turned into a 25,000 seat venue.
The milestone comes as members of the International Olympic Committee's Co-ordination Commission enter the last day of their three-day visit to London.
The 18-strong group has been treated to a series of presentations from London 2012 chairman Lord Coe and Olympic Delivery Authority chairman John Armitt, as well as meeting Olympics Minister Tessa Jowell and the new Culture Secretary Andy Burnham.
Organisers are expected to say their preparations are on track, and will be hoping to gain praise similar to that of the previous inspection 11 months ago.
Work on the 2012 Aquatics Centre is due to start this summer.