But the building products giant is confident of keeping production going despite threats of a shutdown by Unite.
The union said 13 “burners” are threatening to strike and stop the kilns at the plant from Monday April 8 in a dispute over holidays.
Hanson employs about 200 workers at the site.
Unite claim the firm wants to reduce the number of holidays for the burners from 25 to 21-days-a-year.
Hanson transformed the working conditions of everyone working at the site last year after lengthy negotiations produced a settlement on 24/7 working.
A Hanson spokesman said: “When we closed our Saxon works at Whittlesey a year ago and consolidated brick production at Kings Dyke we took the opportunity to introduce harmonised terms and conditions of employment for all departments in the works.
“The new terms, which remain generous and are in line with industry standards, were agreed and accepted by the majority of the workforce who also accepted a one-off payment of £1,500 to compensate for any reduction in benefits.
“It is disappointing that, having accepted the compensation payment, a small section of the workforce has now changed their minds.
“We hope industrial action can be avoided but have a contingency plan in place to ensure the factory continues to run and production is not disrupted.”