Law firm Thompsons has written to Mandelson as part of a consultation process over new blacklisting regulations.
And the firm wants to see offenders face criminal charges - including imprisonment - rather than the current deterrent of civil action.
The consultation was launched by Mandelson in the wake of the construction blacklisting scandal masterminded by Ian Kerr and his company the Consulting Association.
Officials from the Information Commissioners' Office raided the Consulting Association's premises in March and found a blacklist containing the names of more than 3,200 construction workers. More than 40 major construction companies were subscribers to the list.
ADVERTISEMENTKerr was fined just ВЈ5,000 at Knutsford Crown Court for breaching the Data Protection Act.
Thompsons is currently acting for at least 14 construction workers who are looking to sue for damages after being denied work because of the blacklist.
Richard Arthur, head of trade union law at Thompsons, said: "Blacklisting is different to other forms of discrimination, because it is by definition, planned, covert and systematic.
"The Employment Relations Act 1999 which provides the enabling legislation envisaged that there would be criminal sanctions for blacklisting trade unionists.
"It sends the wrong message to potential law breakers to now say that only civil law sanctions such as fines will apply."
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