Workers contacted the Enquirer after being sent letters detailing plans for 10% cuts across their departments.
Costain said the cuts are focused on back office staff and 30 roles have been made redundant from a team of 400.
But sources at the firm fear the cuts could be more extensive.
One insider said: “This started at the back end of last year and they are targeting 10% cuts across departments.
“There are people going at all divisions but rail and head office seem the hardest hit.”
A Costain spokesperson said, “As part of the normal course of business, as you would expect from a successful company, Costain has recently reviewed its overhead cost base to ensure it remains efficient.
“We have 400 people in overhead roles throughout the Group. We have worked to deploy colleagues into other parts of the business, but regretfully, we have had to make 30 roles redundant.”