The two companies currently hold maintenance contracts for the M25, but will lose them after the DBFO deal is signed.
As well as construction work to add capacity to three lane sections of the M25, the Design Build Finance Operate (DBFO) contract also includes maintaining the motorway and its structures for 30 years.
Approximately 500 people are currently employed by Mouchel and Carillion on their Area 5 contracts which have a combined value of around £40M.
"It’s too early to say [what will happen]," said a Carillion spokesman. "We only heard a few days ago."
The Highways Agency expects to award the M25 DBFO contract in late 2008. The Carillion/Mouchel maintenance contract was originally due to finish in September 2006, but the Highways Agency agreed with Carillion to extend it to ensure a proper handover to the new DBFO contractor.