Carillion PLC (LON:CLLN) shares dropped by almost 30% in late afternoon trading on reports that its lenders have rejected the embattled construction firm's business plan, and that it has lined up an accountancy firm as a standby administrator.
The group - which was demerged for Tarmac in the late 1990s - is a major supplier to the government with contracts across education, the NHS and the rail industry, including HS2.
The firm is now under investigation by the Financial Conduct Authority over the "timeliness and content of announcements" made between December 2016 and July 2017.
The RMT said Carillion's workers were "not responsible for the crisis".
Shadow business secretary Rebecca Long-Bailey said the collapse of Carillion could "provoke a serious crisis".
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 needs £300m of emergency funding and lenders are under pressure from the Government to keep the company afloat.
Transport Scotland said that Carillion had "no intention of withdrawing" from the AWPR project, and that the consortium delivering the route "remains committed to completing it in accordance with the contract".
A collapse could be damaging as Carillion holds government contracts in the rail industry, education sector and NHS, and employs about 43,000 people across its global business.
Scottish Labour said it was important to know "just how exposed the Scottish government and the public purse is if Carillion were to collapse".
It is also one of the biggest suppliers of maintenance services to Network Rail and manages schools, roads and prisons.
He said it was a "pretty messy situation", adding: "The situation is pretty bleak".
She added: "The huge pension deficit is a further worry for the Carillion workforce, as well as their jobs being potentially on the line, they have also discovered that their pensions, which they have saved for, could be at risk". The long-awaited route is due to be finished this year following lengthy delays.
Carillion is holding crisis talks with United Kingdom government representatives on Friday, which Sky News said were aimed at safeguarding the more than 28,000 pension scheme members who face potential cuts to retirement payments should Carillion fail.
A Government spokeswoman said: "As Carillion is a major supplier to Government it should come as no surprise that we are carefully monitoring the situation while working to ensure our contingency plans are robust".
Carillion said "constructive discussions with a range of financial and other stakeholders" were continuing.
"It is too early to predict the outcome of these discussions but Carillion expects that any such agreement is likely to involve the outcome of new capital and the conversion of existing financial indebtedness to equity which would result in significant dilution to existing shareholders".