The workers have been in contact with construction union Unite and the Enquirer to share their concerns about working on the London tunnels for Skanska Costain STRABAG (SCS) joint venture.

One veteran tunneler said: “I’ve worked all around the world and in some terrible countries and projects, but this has to be one of the top worst projects on my list.”

Workers have shared photographs of welfare facilities near the tunnel boring machines and vehicles they claim are designed for emergency response on the site filled with rubbish.

Unite has a long-running dispute with SCS over site access and union representation on HS2.

Unite regional officer Declan Murphy said: “We regularly visit the Mace and Align-run parts of HS2 and their sites are spotless.

“It’s time for SCS to allow unions on site and treat their workers with the respect they deserve.”

An SCS JV spokesperson said: “We are disappointed that any claims relayed to the union of this nature have not been reported to us.

“We have an engagement programme to ensure a good workforce culture.

“The health, safety and wellbeing of our workforce is our highest priority. The vehicle pictured by the union is not a dedicated rescue vehicle.

“Whilst it is the responsibility of everyone to treat the welfare facilities with due care and respect, a cleaning team works on a 24/7 basis and attends the TBM to manage the welfare facilities throughout the shifts

“We offer Unite access to our sites to meet our team at inductions, work sites and in our welfare area. To date they have not taken up this offer, however it remains open to them.”

SCS JV said: “The vehicle pictured is not a dedicated rescue vehicle. The frame shown in Unite’s picture can be placed on any of our c.40 vehicles of this type to support a stretcher in an emergency that requires an immediate evacuation and we have multiple options for the recovery of people, including facilities in dedicated vehicles as well as having frames that can placed onto the vehicles pictured.”