Cash said Carillion had paid reduced contributions to the fund since it took it over.
He said: "The 7.5% that Carillion has been paying in is little more than half the level it needs to stump up to keep the fund healthy. Now it has to pay in the full rate, it seems it would rather close the scheme."
He said that rather than taking industrial action over the firm's proposal, he would prefer the government to take action. He said the government ought to ensure that companies took their pension responsibilities seriously by preventing them from underfunding schemes and from setting up poorer alternatives. He said: "But if the government is not prepared to take action, we are – and we will ballot our members for industrial action if necessary."
Carillion was unavailable for comment.