The breakthrough follows a five-year campaign by the unions.

The access agreement was signed this week with joint venture company Skanska, Costain, Strabag (SCS) who are undertaking the work to build the tunnels which will carry HS2 trains in and out of central London.

Under the access agreement union officers will be able to meet with workers in the site rest areas during break times to discuss employment matters.

The unions will also be able to attend induction meetings to talk to workers.

Unite general secretary Sharon Graham said: “The signing of this agreement will ensure that Unite is now fully able to defend and enhance workers’ jobs, pay and conditions at HS2.

“The access agreement is the culmination of a five year campaign to secure standard access to the HS2 projects in central London. It now means that all major parts of the HS2 development are covered by union access agreements.”

Unite regional co-ordinating officer Vince Passfield added: “This has been a long and challenging campaign to secure standard union access to this site. Now that it has been achieved Unite will ensure that workers will have their employment concerns swiftly and properly dealt with.”