About 300 of the 900 staff affected have refused to agree to the new contracts, following concerns that the new terms could leave employees out of pocket, and increase their working hours.
"Our members are willing to change the contract but want further safeguards and assurances," said Peter Skyte, general-secretary of Manufacturing Science Finance (MSF), the union for skilled professional people.
Skyte said he was concerned that the new contracts ask staff to opt out of the working time directive, which limits the working week to 48 hours.
Staff are also unhappy about a clause which gives ICL the right to impose future contract changes.
The services group will meet staff representatives next week, in what it calls "one last effort" to address engineers' concerns. ICL human resources director Peter Richardson said only 20% of staff would be financially worse off. "After having done an industry benchmarking exercise we believe the contracts are in line with market conditions and employees will benefit," the company said.