Weymouth & Portland Academy will be first venue completed for 2012 Games.
Dean & Dyball has started construction work on the national sailing academy for the 2012 Olympic Games.
The contractor, which was bought by Balfour Beatty in March, has started work on a permanent slipway and extended sailing facilities at Weymouth & Portland Sailing Academy.
It will be the first Olympic construction project to be completed.
The ODA said it deliberately held back the start of construction to avoid disrupting the nesting season of local birdlife.
The upgrading of the facilities is expected to be complete in 2008, and the venue will be used for Olympic and Paralympic sailing events in 2012.