Crossrail and the Costain Skanska Joint Venture have also launched an investigation into why dust was forced into the tunnel last week by a ventilation fan as revealed by the Enquirer.

Unite has raised fears over site conditions following the deaths of four workers at home in their sleep this year.

CSJV appointed independent tester 4-Rail Services Limited to conduct an air quality survey which found no issues of concern at the site. Additional air quality monitoring has been undertaken by ParkHealth.

All dust, oxygen and air movement readings to-date have been within required limits while air quality monitoring at the site is ongoing.

Spot measurements for gases which include Carbon Monoxide, Nitrogen Dioxide and Nitrogen Monoxide have also been undertaken throughout the site with levels being well below the documented Work Exposure Limits.

A Crossrail statement added: “On 8 October a tunnel ventilation fan, on another part of the Crossrail project, caused dust to enter the Bond Street site.

“The site team responded immediately and effectively, moving people away from the affected area. The investigation of this event by Crossrail Ltd is ongoing.”

Andrew Mooney, Project Director, Costain Skanska Joint Venture said: “The safety of all colleagues working at CSJV sites is absolutely paramount.

“Independent air quality monitoring at Bond Street has continued this week to offer reassurance to our colleagues that the environment continues to remain safe.

“The results of the monitoring tests are below the HSE threshold values and do not reveal anything concerning or unexpected.

“This situation has been discussed with the HSE which was reassured by our response and is satisfied with the ongoing arrangements with regard to air quality.

“The results have been communicated to our colleagues working on site. Any concerns raised about health and safety on site will always be fully investigated with the appropriate action always taken.

“Work is currently going ahead as normal on site.”

Carole Bardell-Wise, Health & Safety Director, Crossrail Ltd said: “Crossrail Ltd demands the highest standards of health and safety across the project and we continue to work closely with our contractor Costain Skanska Joint Venture in support of making sure this is the case.

“Independent air quality monitoring has confirmed that Bond Street remains a safe environment. The issue of air quality is taken very seriously by everyone in the construction industry.

“Crossrail and its contractors monitor the air quality at all sites to ensure that compliance requirements are met and that workers are not exposed to unacceptable levels of dust.

“Crossrail will always ensure that its contractors are focused on the use of health and safety control measures which place the emphasis on the elimination of hazards at source.”

A site source told the Enquirer: “Since the new CSJV management team has taken over, site conditions have improved, and the incidents over the past couple of weeks have resulted in them listening to us more.”