The experts claim that firms could be hit by fines of up to £50,000 if they have not signed up to a scheme by the government's 8 October deadline.
Insurance firms suggest that the problem is prevalent among small and medium-sized contractors.
Under new government legislation, all companies that have more than five employees have to provide access to a stakeholder pension scheme.
Figures released by pension provider Virgin Direct reveal that only 50% of construction companies have signed up to a new scheme.
Brian Griffiths, chief executive of construction pension provider B&CE, said although the firm had signed up most of the top 200 contractors, small and medium-sized firms had been slower off the mark.
He said: "Many medium-sized contractors could face fines of up to £50,000 if they are not signed up by the October deadline, and our representatives are still meeting with firms in order to sign them up before the deadline."
The Virgin Direct research shows that 31% of construction firms will not get a stakeholder pension until the last minute, and only 38% intend to make an employer contribution.
Gerry Lean, director of industrial relations at the Construction Confederation, also warned that there were many firms that would face fines unless they acted quickly.
He said many of the B&CE stakeholder pension holders were automatically switched from existing lump sum-style schemes, but it will still leave many in the industry uncovered.
He added: "The majority of uncovered contractors will be heading for massive fines unless they sort out a scheme within the next few weeks."
Barry Stephens, deputy chief executive of the National Federation of Builders, said it was a problem smaller contractors must take seriously.
He said: "Firms can't afford to be complacent. We will be reminding contractors that there is only one month to sign up and placing a reminder advert for them in our members' magazine."