The Union Ħaddiema Magħqudin will call for Infrastructure Minister Austin Gatt's resignation if a report detailing the way the water and electricity tariffs were worked out, is found to be misleading.
"This is a whole mess mastered by Austin Gatt," union general secretary Gejtu Vella said.
He added that the process of social dialogue had failed and the water and electricity tariffs issue was completely mishandled.
The Malta Resources Authority is expected to explain its impact assessment report to the Malta Council for Economic and Social Development in the coming days, and Mr Vella said the union would be asking the regulator to look into the issue in detail, including the present tariffs.
"If it transpires that the bills are wrong, Minister Gatt should resign," he said.
The saga, which has been ongoing since October, took a new twist on Friday with conflicting reports on whether the authority would be revising the bills, which have already been issued.
In the past weeks, the UĦM had taken a back seat as 11 other unions battled against the tariffs, because they had come out in favour of the final package.
While the 11 unions said they were promised backdated adjustments by the authority, the regulator categorically denied this.
In fact, MRA chairman Carmel Ellul was uncertain whether the regulator had authority over the issued bills, saying that retroactivity was not mentioned in the MRA's law.
Meanwhile, Prime Minister Lawrence Gonzi yesterday completely ruled this out: "The MRA does not have the authority to retroactively change the tariffs."
However, he told The Sunday Times, if mistakes were found by the MRA, these would have to be corrected.
Asked what would happen if mistakes were to be found in the tariffs dating back to October 1, Dr Gonzi insisted the authority had already carried out studies which show there were none.
"It (the MRA) has done its projections and presented them in detail. These are all non-existent theories. The MRA has carried out its studies and the exercise was correct," he said.
The unions said the MRA told them its report was finalised two weeks ago, whereas the legal notice that brought into force the new tariffs was published in December.
When it was pointed out that the MRA had not been involved from the start, Dr Gonzi said it was irrelevant when the authority came in, whether at the beginning or the end.
Asked whether people should pay their bills, Dr Gonzi said: "Of course they should pay their bills. Whoever suggests that bills should not be paid is irresponsible, because at the end of the day it is the consumer who will suffer."
On Friday, the 11 unions called on people not to pay until they received a revised bill, with teachers' union head John Bencini saying that if these took six months to be issued, people should not pay for six months.
Yesterday, the unions stuck to their guns, and called for the imminent resignation of authority chairman Carmel Ellul.
In a joint statement, they insisted that the MRA had promised them a backdated revision, with Mr Bencini saying he had put the question to the authority four times, and was told this would be the case.
The unions said they were shocked by the MRA's denial, and in the coming days will be going to court to take an oath about the meeting's outcome.
On the other hand, MRA kept insisting it had not agreed, recommended, or tried to give the impression that it would be carrying out a backdated revision.
It said some union representatives might not have understood how a revision of tariffs is carried out, and assumed the authority would be directly revising bills which have already been sent.
The MRA also rejected the union's proposal to call for bills to stop being issued until the revision is carried out. The authority said it was prepared to meet the unions again for further explanations.