Construction union UCATT has expressed its "severe disappointment" that government blacklisting regulations have not been strengthened despite highlighting concerns they were not sufficient to end the practice.
The regulations failed to make blacklisting a criminal offence, despite the government promising in 1999 when the original legislation was passed to introduce such measures.
They also specifically rule out protecting workers involved in "unofficial industrial action."
Workers who down tools due to safety concerns or who refuse to undertake overtime could be legally blacklisted.
Automatic compensation for any blacklisted worker is also not covered in the proposed legislation.
Furthermore, in certain circumstances, companies will still legally be able to draw up lists of trade unionists for recruitment purposes, a practice which campaigners have described as back-door blacklisting.
The proposals were laid before Parliament on January 5 and are due to be debated by the joint committee on statutory instruments tomorrow.
UCATT has written to members of the joint committee, outlining how, in its view, the regulations are deficient.
UCATT leader Alan Ritchie said: "The blacklisting regulations are fundamentally flawed.
"It is deeply disappointing that the government has failed to listen to the genuine concerns of trade unionists and have failed to make the appropriate amendments to the regulations."
The regulations stem from the case of Ian Kerr and his firm the Consulting Association.
Mr Kerr was found to be in possession of an illegal database of 3,200 workers which identified communists, union shop stewards and other "troublemakers" which he sold to construction firms.
He was convicted of breach of data protection laws and fined £5,000.
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