But the protest plans have left the contractor baffled after workers were asked whether they wanted to talk to UCATT – and said no.

The union is also claiming that Shepherd is employing workers as subcontractors via umbrella agencies rather than directly.

A Shepherd spokesperson told the Enquirer: “Trade unions are welcomed on any of our sites and UCATT is no exception.

“The feedback we receive from workers is that impromptu visits can be disruptive so, as previously discussed with UCATT, we respectfully request that all visits are prearranged to fit with our construction schedules onsite.

“Each month, such requests are discussed with all workers – both direct employees and sub-contractors – and are scheduled according to their preference.

“As a specific reason for UCATT’s visit had not been stated by its representatives, workers onsite have so far stated a preference not to host a site visit.

“We are in constant dialogue with onsite workers and, should this situation change, we would of course facilitate a meeting.

“It is common practice in the construction industry to subcontract services. It is our policy, however, not to use umbrella companies to pay Shepherd employees.

“All of our agreements with subcontractors require them to operate in both a legal and compliant manner.”

UCATT is planning a demonstration at midday outside the site.

Mark Martin UCATT Regional Organiser, said: “What have Shepherd got to hide? By barring UCATT from the site, they are clearly worried about something.

“If they haven’t got anything to hide they should allow the union access to speak to workers.”