Union leaders have threatened to take legal action against a waste company it claims is trying to find staff to work during planned industrial action.
Unite members employed by Suez, in Doncaster, are to stage two five-day strikes in a row over pay.
The union say Suez is "actively recruiting agency workers to try to break the strike".
Suez said it was not responsible for placing an advert for staff which stated it was to cover strike action.
The two strikes are set to start on 23 August and 2 September.
Under the Conduct of Employment Agencies and Employment Businesses Regulations 2003 it is illegal to employ someone to perform "the duties normally performed by a worker who is taking part in a strike or other industrial action".
The advert posted by Aim Recruit Ltd on 8 August stated: "This work is to cover industrial action and the workers will need to cross a picket line."
Regional officer Shane Sweeting, said: "Unite will leave no stone unturned, be it legal, political or industrial in ensuring that Suez is not allowed to break the law."
'Misleading and erroneous'
The strike was called in a row over pay, however Unite said it intends to ballot members again after claims Suez plans to axe 100 jobs.
A spokesman for Suez said: "Although Suez was not directly named in the advert, it clearly creates the misleading and erroneous impression that it was placed on Suez's behalf".
He said that the recruitment agency was unable to say who had commissioned the advert and it had been withdrawn.
He said: "We are therefore at a loss to explain who instigated the advert, or why, but we will be investigating further and will take a robust approach to dealing with this issue."
He added that Suez respected union members' right to strike but it was not illegal to recruit temporary workers to "to assist in the catch-up following any strike action".
Suez said any redundancies would be a result of changes to its contract with Doncaster Council.
It said it had begun a formal 90-day consultation period with staff.