A number of staff are also facing the axe in Leeds and Newcastle as contracts come to an end.
Balfour confirmed last year that it would stop bidding new M&E work in London and the South east after the division racked-up an £88m loss in 2014.
A Balfour satement said: “This week we commenced redundancy consultations with 35 mechanical and plumbing skilled operatives working in our Engineering Services business in London and the South East.
“This is as a result of our decision not to pursue work in the London and South East area as a Tier 2 subcontractor as well as a number of our historic contracts in this difficult market coming to an end.
“We are also in consultation with a small number of monthly paid staff and skilled operatives in our mechanical and plumbing workforce in Leeds and Newcastle due to a number of projects having completed.
“We are looking to redeploy impacted individuals wherever possible.”