Under a new scheme being backed by the government, union safety representatives will no longer be confined to their own sites.
It is unclear whether these officials will have the power to close an unsafe site, but they will be able to give advice on safety.
The scheme, to be co-ordinated by the TUC, will involve representatives from UCATT, TGWU, GMB and the AEEU. It is being drawn up by the Construction Industry Advisory Committee, a body that comprises representatives from the Health and Safety Executive and the industry.
The scheme is understood to enjoy the support of the HSE and union officials, but its construction members are opposed to it.
Deputy prime minister John Prescott is understood to be behind the plan. He has repeatedly criticised construction's safety record after deaths rose 20% in the year to April.
Suzannah Thursfield, Construction Confederation director of health and safety, said the introduction of roving safety representatives would create ill-feeling on site.
Inviting a stranger on site will throw up barriers and lead to a lot of mistrust
Suzannah Thursfield, Construction Confederation
She said: "We recognise that employers need to consult workers more but inviting a stranger on site to try to represent you will throw up barriers. It will lead to a lot of mistrust between employers and trade unions and between workers on site and employers."
Thursfield said the scheme would also lead to extra costs. "At present, workers do their union duties alongside the day job. You are asking employers to pay for something they don't believe is the solution," she said.
A source close to the scheme said the confederation was complaining because it did not want to lose its control over sites. He said: "The last thing it wants is anyone but itself and site management having an input on site. It is about protecting the status quo no matter how bad."
Studies by the Health and Safety Commission and DETR have found that sites with union safety reps are safer than those without.
The CONIAC advisory committee's occupational health working party is discussing details of the proposal at a meeting next week.
The Major Contractors Group is also thought to be opposed to the scheme. A source close to the group said: "We've been asked to comment. We will probably be against it."