O’Rourke chief executive Anna Stewart hit back at Ucatt in a strongly worded letter sent to MPs and the Mayor of Liverpool who have been lobbied by the union.

Ucatt regional officials claim they have been barred from the job and O’Rourke workers who are union members are being kept off the site.

That is totally denied by the contractor who have taken the unusual step of putting their side of the argument publicly.

Stewart said: “We feel compelled to reply to these falsehoods in detail to protect our good reputation in industrial relations and safety management, and to set the record straight.”

The Ucatt allegations follow a recent appearance by general secretary Steve Murphy at the Scottish Affairs Select Committee into blacklisting.

One rival contractor told the Enquirer: “There will be a lot of firms happy to see O’Rourke stand up to Ucatt over this.

“The unions seem to think that since the blacklisting scandal they can get away with chucking any sort of mud at major contractors and they will be too cowed to fight back.”

O’Rourke bosses are furious about the accusations that they are anti-union and anti-safety.

The company said: “Nothing could be further from the truth.”

The Alder Hey site has suffered a recent spate of three reportable accidents.

Ucatt claim the “lack of safety representatives is thought to be a major factor in a spate of accidents on the site.”

But O’Rourke believes union safety reps do not have the technical skills to improve their own systems.

The company said: “To suggest that the lack of union safety representatives could be a factor is untrue.

“It is our belief that we will create a safer working environment by working together and encouraging everyone to focus on safety.

“On this point, whilst a union appointed safety representative who visits the site periodically can certainly help to promote the importance of safety on site, we do not believe it will add anything beyond our existing extensive approach.

“Furthermore, they do not possess the technical understanding of our unique delivery approach to fully comprehend and be able to mitigate the safety risks on our projects – therefore we simply cannot countenance their involvement in this critical element of our inductions.”

Ucatt also claimed that “large numbers ” of the workforce are being employed via labour agencies

O’Rourke said: “Laing O’Rourke has the largest directly employed operative workforce within the UK building and engineering industry.

“Alder Hey currently has a workforce of approximately 450 site-based workers. Of that number, around 340 are directly employed by Laing O’Rourke companies.

“Where we have a short term or temporary need for labour, we will engage with employment agencies.

“Laing O’Rourke currently has approximately 12 people employed on Alder Hey via reputable agencies that have pre-existing union agreements in place with Ucatt.”

One industrial relations expert said: “It shows how out of touch with reality the unions are if they are having a crack at O’Rourke over direct labour.

“Everyone knows O’Rourke has the largest directly-employed workforce in the business and has been a champion of direct employment for years.”

To read a full copy of Anna Stewart’s letter click here