The men were given their cards on a ВЈ180m power plant near Slough being built by Japanese contractor Takuma.
The workers claimed they had not been adequately consulted by Fabricom before being shown the door.
One said: "This came completely out of the blue. Most of us affected have been working here for at least six months and there is still plenty of work to do.
"We're not going to stand for this and will be taking them to a tribunal."
Fabricom's Human Resources department declined to comment. A site source for the firm said: "There has been plenty of consultation over this and we have gone through the right procedures.
"Labour needs change all the time on a big job like this."
The redundant workers are now consulting with union leaders at Unite and the GMB.
One said: "Most of the people shown the door were union members and won't take this lying down."
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