The state government insists the Melbourne Metro Rail project is on track and within budget despite concerns it is facing a major cost blowout and could be delivered up to a year late.

It is unclear whether the government will have to stump up more cash for the project.

Infrastructure and market sources aware of the project's problems said it was expected that three private companies building the tunnel would wear the majority of the cost blowout by raising more debt.

Transport Infrastructure Minister Jacinta Allan Credit:Jason South

The Age in June reported the Melbourne Metro Tunnel project could exceed its $11 billion budget by up to $2 billion but sources believe it could be higher.