Carillion - the UK's second-largest construction company - is engaged in crunch talks with the UK Government and the Pensions Regulator in an attempt to avoid going into administration.
Shadow business secretary Rebecca Long-Bailey said the collapse of Carillion could "provoke a serious crisis".
It said the firm remained in constructive dialogue about short term financing while "longer term discussions are continuing".
A spokeswoman said: "It is too early to predict the outcome of these discussions but Carillion expects that any such agreement is likely to involve the raising of new capital and the conversion of existing financial indebtedness to equity which would result in significant dilution to existing shareholders". It is involved in projects such as the HS2 railway line and in maintaining sites for the Ministry of Defence. It employs 43,000 people globally.
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".
The firm's troubles have prompted calls for the government to bring its contracts back into public control amid fears it could "collapse".
The government confirmed ministers met yesterday to discuss Carillion's future and were "monitoring the situation closely".
Ministers reportedly discussed the contingency plans in place should Carillion collapse, news of which first emerged on Wednesday during Cabinet Office orals.
The company has been pushed to the brink by cost overruns that forced it into a string of profit warnings a year ago and left it on course to breach the terms of its bank loans.
Adding to the pressure on the small cap firm - which issued three profit warnings in less than six months previous year and has seen its market value collapse by 90% - was a recommendation from broker Peel Hunt to sell the stock ahead of forthcoming newsflow.
Unite assistant general secretary Gail Cartmail, said: "The government must consider all options while the future of Carillion hangs in the balance, including bringing contracts back in-house".
A Scottish Government spokesman said: "We continue to liaise with UK Government colleagues to monitor and mitigate service risks associated with Carillion's financial situation and stand ready to offer what assistance we can at this anxious time for the company's employees and their families".
"We will not comment further unless it becomes appropriate to do so".