Carillion, who are contracted to build HS2 from Aylesbury to Leamington Spa, have collapsed.
The construction giant is in a partnership which won two HS2 deals worth £1.34bn.
Carillion won a total of 10 separate contracts worth over £1.3bn after issuing last July’s profit warning that kick-started the chain of events leading to its ultimate demise.
To make matters worse, two contracts, with a total value of £137m, were awarded after Carillion’s second profit warning on 29 September, at which point the company said that it might have to sell shares to bolster the health of its balance sheet.
This included a 50-mile section of the high speed railway roughly between Aylesbury and Royal Leamington Spa.
Carillion and the two other firms in the joint venture - Eiffage and Kier - were forced to give assurances that they could step in to deliver on the work if one of the partners collapsed.
A HS2 spokesman described Carillon going into liquidation as “clearly disappointing for them and the wider UK construction industry”.
He went on: “We are continuing to discuss with Kier and Eiffage the implementation of contingency plans.
“Work will continue as planned with no unnecessary or additional exposure to the taxpayer.”
Phase one of the £55.7bn railway will run between London and Birmingham from December 2026, with a second Y-shaped phase extending to the North.
Carillion are one of around six main contractors for Network Rail, which owns and operates Britain's railway infrastructure.
It is mostly involved in bridge replacements and track renewal, with contracts worth in total around £150m to £200m.
The government’s Insolvency Service urged Carillion’s 19,500 UK staff to go to work as usual and assured them they would get paid to continue providing services such as school dinners, hospital cleaning and prison maintenance.
The prime minister’s official spokesman said that some of Carillion’s 450 public sector contracts could be taken in house, although that was “a decision for further down the line”
Contracts for building part of the HS2 rail link will remain in the private sector, he added. Kier and Eiffage, the other two construction partners, have assured ministers they can build the London to Birmingham section of the line without Carillion.