The women won a landmark£1m case after their wages were cut in a bidding war for services. Men doing similar jobs did not have their wages cut.
Unison and GMB won settlements worth about£1,300 for each woman as industrial tribunals against the now defunct Cleveland CC were about to start.
Now 1,500 of the workers have begun fresh actions centred on productivity bonuses that could net them£8m.
The four new councils which employ the women said jobs could be at risk if they have to pay out, reports the paper.