Plans to speed up journeys by electrifying the main Edinburgh-Glasgow rail line have been delayed again, transport minister Humza Yousaf told MSPs today.
He announced Network Rail had said a "safety-critical" component had failed, which would affect the planned start of electric trains in July.
The news came as the organisation revealed the performance of a project group that includes construction firms had "not been acceptable".
The setback follows the original completion date being put back from last December because of other difficulties with the so-called Edinburgh Glasgow Improvement Project (Egip).
The problem will delay overhead electric power cables going live - or "energisation" - at the end of this month.
That will have a knock-on effect on ScotRail drivers being trained to drive electric trains on Scotland's busiest route.
Industry sources told The Scotsman that the latest problem could also postpone the start of an all-electric service of brand new trains on the line from December.
It is understand the faulty equipment is part of the overhead power lines may have to be replaced over the entire 46-mile route.
Mr Yousaf told MSPs any further delay after the earlier problems announced last summer would be "extremely disappointing".
A Scottish Government spokesman later described the situation as "wholly unacceptable".
The £742 million project will enable faster-accelerating, longer and greener electric trains to replace diesels on the route via Falkirk High.
Electric trains from other routes are planned to initially take over some services by the end of July.
They were due to be followed by the first of a fleet of brand-new Japanese Hitachi trains from September, with the last diesel trains replaced in December.
They will be progressively lengthened between December and the end of next year, when Edinburgh to Glasgow journeys are due to be cut by about ten minutes to 42 minutes.
Mr Yousaf said the delay further highlighted the need for the devolution of control over Network Rail so the Scottish Parliament could properly hold it to account over its projects and spending north of the Border.
The news came in a letter to the minister from Network Rail chief executive Mark Carne on Monday.
It said: "Regrettably, it is now clear that a safety critical component is susceptible to failure and must be replaced.
"This will impact the energisation start date.
"We are working extremely closely and collaboratively within the ScotRail Alliance to assess how the impact of this challenge can be minimised for passengers and we will keep your officials fully informed."
Further Scottish Government concerns about the project are referred to in a separate letter from Mr Carne to Transport Scotland chief executive Roy Brannen,
Mr Carne said he had been "personally addressing other challenges" where "performance has not been acceptable and action is being taken".
These involve the Egip Alliance, which includes construction firms Costain and Morgan Sindell.
Mr Carne also referred to a "timetable dispute", which The Scotsman understands relates to Network Rail's plans to lengthen journey times so trains are more likely to arrive on time.
He said Network Rail's performance targets included train reliability but not journey time.
A Scottish Government spokesman said: “The current situation is wholly unacceptable.
"Network Rail, which is under the control of the UK Government’s Department for Transport, is in the process of confirming the impact of this latest update on the delivery of electrification.
“We have asked Network Rail to identify the best solution with a clear focus on the needs of passengers and the priorities of Scottish ministers.
"Once again, this situation highlights the need for further devolution of Network Rail.”
Scottish Conservative transport spokesman Liam Kerr said: "This is a project that has not been without its challenges.
"Rail passengers will be dismayed to hear of a further delay.
"Travellers simply want to see improved services, and will care little for the machinations of what has gone wrong.
"It’s crucial both the Scottish and UK governments work together to make sure this project gets over the line.”
A Network Rail spokesman said: “There is an emerging issue around some of the installations on the Edinburgh-Glasgow electrification programme.
"We are working hard to assess the implications of this and won’t be commenting further until we have completed that work."