Roads minister Tom Harris made the announcement yesterday.
The scheme includes the replacement of mechanical and electrical equipment, and installation of new fire safety equipment.
Work is due to begin in autumn 2008 and is expected to be completed by December 2009.
Harris said: "Awarding the contract is an important step in taking forward this important project, which will ensure the ongoing safety of Bell Common Tunnel - constructed back in 1983."
120,000 drivers typically travel through the M25 Bell Common Tunnel every day.