Unite says it has evidence that private waste contractor Suez is actively recruiting agency workers to try to break the strike and added: "The recruitment of agency workers to break or undermine strike action is strictly illegal and is a criminal offence."
The first strike is scheduled to begin on August 23 until August 27 and a further period of strike action will then occur from September 2 finishing on Wednesday 6 September.
But Suez has blasted the claims and says that is staying within the law over rules regarding industrial action.
Unite says it has seen an advert circulated by Aim Recruit Ltd - which already supplies agency workers to Suez - stating: “This work is to cover industrial action and the workers will need to cross a picket line."
Other agencies identified to be recruiting for refuse workers on the contract include Logical Personal Solutions and Ideal Recruit.
Unite is writing to the government’s Employment Agency Standards Inspectorate, which is responsible for ensuring that the rules on agency workers are upheld, warning that Suez and the agencies concerned are prepared to act illegally.
Unite will also write directly to the company and the agencies warning them that they will be engaging in illegal activity.
Unite regional officer, Shane Sweeting, said: “Suez’s actions are disgraceful and the company will be acting illegally if it recruits agency labour to undermine a strike. It is simply incredible that Suez is more interested in recruiting agency workers to try to undermine the dispute rather than entering into negotiations with Unite.
“Unite will leave no stone unturned be it legal, political or industrial in ensuring that Suez is not allowed to break the law”.
The initial dispute concerns pay and Suez has offered workers a two per cent increase but this is conditional on the removal of guaranteed overtime, which will in reality mean that workers will have their pay frozen.
The Suez dispute has further deepened as the company has also announced that it is looking to effectively halve the 250 strong workforce by October and is intent on making over 100 workers redundant.
Mr Sweeting, added: “If Suez gets its way Doncaster will become permanently dirty. There will be simply not enough staff to keep the town clean.
“It is imperative that the council and the local politicians put pressure on Suez to enter into sensible negotiations and not press ahead with slashing the service.”
A spokesman for Suez said: “A recruitment advert was brought to our attention earlier this week and we can confirm that, following internal investigations, Suez had no involvement in the advert or its placement.
"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.
"When Suez challenged Aim Recruit Ltd about the origin of the advertisement, they would not confirm who commissioned it and it was quickly withdrawn from publication. 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.
"Suez respects the right of Unite members to engage in legal industrial action and we are fully aware of our obligations and limitations as an employer in these situations. For the record, however, Suez may make use of agency workers to assist in the catch-up following any strike action, which is in accordance with the relevant legislation, but we will not employ any additional agency staff to conduct work during stoppages should they occur.”
Jo Miller, Chief Executive of Doncaster Council, said: “This advert which was brought to our attention was clearly of great concern as it was advertising something unlawful.
"We immediately contacted the company who carried out their own investigations and assured us that they were not involved in placing the advert and it was taken down from the agency’s website straight away. Suez are continuing to investigate to find out who was responsible.
“What we need now is for the relevant people to get around the table with ACAS to come to an agreement so that the people of Doncaster don’t have to experience another strike.”