Unite is stepping up its search for information into the possible collusion by trade union officials into the blacklisting of construction workers.

The union established an independent inquiry last year into allegations that some union officials may have colluded with the blacklisting.

A legal team has been instructed to investigate whether any union officials from Unite or its predecessor unions were involved.

The inquiry is now entering its next stage, with the launch of an online portal to allow anyone who has any information relating to the inquiry to submit information.

Unite general secretary Sharon Graham said: “Since becoming general secretary of Unite, I have made it completely clear that no stone will be left unturned to obtain conclusive proof whether any current or former union officials were involved in the blacklisting of workers.

“Blacklisting is a disgusting practice which ruins workers’ lives. Unite has been and continues to be at the forefront of stamping out this practice once and for all.

“That is why the inquiry into any collusion between union officials and blacklisters is so critical.”

In 2009, a raid on an organisation called the Consulting Association discovered it had been blacklisting thousands of construction workers, leaving many of them unable to find work again in the construction industry.