Jim Currie said that he had 'pushed the cause of retaining Council control over joint ventures as far as [he could] with the Cabinet' and that the financial risks involved with the 'rush' into strategic partnerships were 'unacceptable'.
Outsourcing of the council's services could go to either BT or CSC if the plans go ahead in a deal that is worth up to £300 million a year.
In September last year councillors voted to postpone the signing of contracts, but cabinet members forged ahead regardless.
However, a successful petition has resulted in plans for a full-council debate, which will take place on 23 October.
Alec Robertson also faces a vote of no confidence on 16 October.