A union is taking legal action to seek compensation on behalf of workers it claims were blacklisted by one of the UK's biggest companies.
The GMB accused services giant Carillion of blacklisting hundreds of workers across the UK, saying the case was the construction industry's equivalent of phone hacking by newspapers.
The union published a report at its annual conference in Brighton which it said pulled back the "curtain of secrecy" to reveal the way firms like Carillion denied workers their rights to employment.
Thousands of workers were unaware that they had been blacklisted by companies, according to the report.
The GMB revealed it had instructed a law firm to seek compensation for its members blacklisted by Carillion.
Paul Kenny, general secretary of the GMB, said: "This report pulls back the curtain of secrecy to give a glimpse as to the way employers like Carillion have illegally used their power and money to blacklist citizens and to deny them their rights to employment."
Carillion said in a statement: "Carillion does not condone or engage in blacklisting. It is not against unions and recognises trade unions for some of its workforce nationally.
"These allegations of blacklisting are believed to concern matters that took place many years ago and relate to businesses acquired by Carillion.
"The law has changed significantly in the intervening period. Nonetheless, Carillion takes any allegation of this nature very seriously and is investigating.
"As these matters are also the subject of threatened legal proceedings, it is not appropriate to comment in more detail at this stage except to confirm that Carillion will robustly defend any such proceedings."