Unions have clashed with Southampton City Council over its pay for temporary child care staff.
Unison said it received a leaked email from a recruitment agency, offering £230 per day to social workers.
Council social workers' salaries begin at £26,000 - about £100 a day - and the disparity is "forcing experienced social workers to leave", Unison said.
Councillor Jeremy Moulton said: "We will not take risks with vulnerable children."
Andy Straker, Unison representative, said: "The council is forcing experienced social workers to leave the council because it is cutting their pay, and then paying self-employed social workers up to double what it pays its own staff.
"If the Conservatives needed another reason to re-think their plans to cut council staff wages from 11 July, then surely this is it."
The leaked email stated the council was looking for "eight qualified social workers and two senior practitioners to work within the assessment teams and long term teams in the council.
"These posts are to start immediately."
In the leaked email, qualified social workers were offered £230 per day and senior practitioners £250 per day. The higher figure equates to an annual salary of about £65,000.
By contrast, directly employed social workers are paid between £26,000 and £34,000, with senior practitioners able to earn up to £38,000 a year.
Mr Moulton said: "We are expanding our social services capability and recruiting more social workers, to address a staff shortage in this area.
"We will not take risks with vulnerable children.
"With the current work to rule conditions imposed by the unions many of our staff cannot cover for vacancies, as well as other duties.
"We would call on unions to make sure working to rule does not impact upon the safety of our children."
Unions are currently carrying out strikes against a package of measures estimated to cut about 5.5% from pay.