The Olympic Stadium will be the centrepiece of the Games
Building work on London's Olympic stadium will begin in two weeks' time, the BBC has learnt.
The £496m stadium in east London will start to take shape on 22 May.
The 80,000-seat stadium will be the centrepiece for the 2012 Games and will host the opening and closing ceremonies as well as the athletics events.
Once the Olympics are over, it will be turned into a 25,000-seat community venue and could also become home to a lower league football or rugby club.
The construction work is about three months ahead of schedule as work to clear the site has progressed quicker than expected, the BBC's sports correspondent James Pearce said.
He added: "Building work was due to begin in August so they are three months ahead of schedule.
"It may sound impressive, but many people will say they need these extra three months as it is going to be a fairly tight schedule, because this has to be a stadium ready to host the Games in four and a bit years.
"We all saw what happened with Wembley Stadium - projects can be delayed, projects can go wrong.
"The construction will be very much be in the public eye. The ultimate embarrassment will be that it is not ready on time."
The Olympic Delivery Authority (ODA) said many buildings had been demolished and 800,000 tonnes of contaminated soil had been removed since the clearing work in the Lea Valley area began in July 2007.
It added plans for a ceremony to mark the start of building work would be confirmed closer to the date.
Currently a huge bowl has been dug and final tests are being carried out to check the foundations.