Workers at troubled construction firm Carillion should have their jobs safeguarded and the government should consider bringing its contracts back under public control, unions have said.
Carillion is struggling under £1.5bn of debt, including a pension shortfall of £587m, raising fears about its future.
The government says it has been monitoring the situation to ensure its "contingency plans are robust".
It is holding a meeting on Friday to discuss the firm's pension deficit.
Carillion is the UK's second-largest construction company and employs 43,000 people globally.
It has been awarded contracts to build part of the £56bn High Speed 2 railway, including the first phase of the line which will run between London and Birmingham and is scheduled to open in 2026.
The general secretary of the RMT rail union, Mick Cash, said Carillion's workers were "not responsible for the crisis".
He added that workers "should have protection and guarantees from the government, including an assurance that operations will be directly transferred over to Network Rail with all jobs, pensions and rights safeguarded if Carillion goes bust".
"RMT has been through a similar situation to this with the collapse of both Metronet on London Underground and Jarvis on the railways.
"This is the high-risk gamble you take with handing infrastructure over to speculative private companies and the workers caught in the crossfire must be protected."
In addition to its rail operations, Carillion also manages nearly 900 schools, provides services to the NHS and works with National Grid.
The assistant general secretary of the Unite union, Gail Cartmail, said: "The government must consider all options while the future of Carillion hangs in the balance, including bringing contracts back in-house."
The company held talks with its lenders and advisers in London on Wednesday.
However, no announcement has been made on a business plan to secure its future.
The Financial Times reported that ministers from across a number of departments met on Thursday to discuss Carillion's financial problems.
It said that David Lidington, who was moved to the Cabinet Office as part of Prime Minister Theresa May's reshuffle this week, convened a meeting with Business Secretary Greg Clark, new Justice Minister Rory Stewart, new Transport Minister Jo Johnson and Liz Truss, Chief Secretary to the Treasury.
A spokeswoman for the government declined to comment on any specific meetings.
"Carillion is a major supplier to the government with a number of long-term contracts. We are committed to maintaining a healthy supplier market and work closely with our key suppliers," she said.
"The company has kept us informed of the steps it is taking to restructure the business. We remain supportive of their ongoing discussions with their stakeholders and await future updates on their progress."
Carillion was forced to ask its banks, which include Santander UK, HSBC and Barclays, for support after breaching its loan agreements last year when it issued a series of profit warnings.
The firm's share price has plunged by more than 90% over the past year.
The company has been working on a plan which it said "will provide the basis for the agreement of a proposal to restore Carillion's balance sheet".
A spokeswoman for the Pension Protection Fund said it was "aware of the discussions between the company, government and banks and, along with the trustees and the Pensions Regulator, will act as it always does to protect the interests of Carillion scheme members and levy payers".
A spokesman for the Pension Regulator said: "We have been and remain closely involved in discussions with Carillion and the trustees of the pension schemes as this situation has unfolded. We will not comment further unless it becomes appropriate to do so."