The union spoke out as councillors were told by the Mears that it would ensure the council was repaid in full after a building sub-contractor overcharged for work by more than £300,000.
GMB regional organiser Gary Palmer said: “The GMB are shocked by the way the council seem happy to simply accept a £300,000 repayment without insisting on commissioning an independent investigation and audit of the entire contract to ensure no other public money has found its way to Mears incorrectly.
“There is a potential that this overcharging scandal to be much more widespread then everyone is currently admitting to.
“It was simply never in Mears’ best interest to look closely at any of their sub-contractors charges to see if they are correct as they make their money in these cases by adding on a percentage as an uplift before billing the council for them.
“It’s a case of it being a publicly funded fruit machine for them and one which always pays out.
“What it does though is raise many questions, collusion or incompetence being one potentially, and should Mears continue to provide this service to the council being another?
“Enough is enough! Cancel Mears’ contract as their reputation is itself now beyond repair and bring services and staff back in-house and under direct council control on both standards and cost while undertaking a full and comprehensive independent investigation and to look to recover all over payments if identified.”
The council said that various audits were taking place and that Mears had pledged to pay for the process.