The meeting is part of the government's review of employment terms throughout industry, but will deal specifically with construction.
Paul Corby, construction head of M&E union Amicus, who will attend the conference, said: "Construction unions will present a united front, calling for government to change legislation to clamp down on bogus self-employment."
He said he would also be asking the government to tackle the tax scam used by agencies that involves setting up "composite companies".
Bogus self-employment and composite companies are devices used by workers and agencies to pay a reduced rate of tax and National Insurance.
Jerry Lean, industrial relations director at the Construction Confederation, said he would tell government officials to make greater use of its present range of sanctions. He said: "The government needs to recognise that it should enforce its policies to root out improper practices by labour agencies."
Birse group industrial relations director Phil Harris urged the government's employment review to safeguard the working practices of professional workers.
He said he would ask for clarification of what the DTI is seeking in the review, as there was uncertainty about what it wanted to achieve. Harris said he feared that legislation aimed at unscrupulous agencies supplying unskilled labour could hit other targets unintentionally.
He said: "I want to make sure wide-ranging reforms do not hinder professionally self-employed people, whom many contractors rely upon."
It is expected that the government will consider the Health and Safety Executive's consultation document on safety law, which addresses the issue of labour agencies, in the review.
The HSE consultation document says contractors must identify the skills, experience and qualifications required for jobs. It also notes that workers must also provide agencies with accurate information about their skills, experience and qualifications so that the agencies can match contractors' requirements with those of workers'.