Passengers will not be able to travel by electric train on the main Edinburgh-Glasgow line until October, transport minister Humza Yousaf announced today.
The delay - nearly a year later than planned - confirms an exclusive Scotland on Sunday story that the project had been put back by another three months.
Mr Yousaf also hinted to MSPs that the £800 million cost, which has increased from £650m, could rise further.
The first electric services on the main route via Falkirk were originally due to have started last December, but were then postponed until July.
The latest delay is because faulty electrical equipment will have to be replaced along the 46-mile route.
Delays hit electric train plans for Edinburgh-Glasgow line: http://www.scotsman.com/news/transport/delays-hit-electric-train-plans-for-edinburgh-glasgow-line-1-4472211
Mr Yousaf said the overhead cables which power the trains will not now be switched on until September - four months late.
He has demanded there is no delay to the start of all-electric services on the line from December, and journeys being cut by ten minutes to 42 minutes in December 2018.
However, the minister dodged questions about whether the project cost had increased, saying that was for Network Rail.
However, Scottish Conservatives West Scotland MSP Jamie Greene accused him of "passing the buck".
Network Rail chief executive Mark Carne and ScotRail managing director Alex Hynes will tomorrow appear before a Holyrood committee to answer questions on the project.
Scottish Conservative transport spokesman Liam Kerr said: "People will be incredibly frustrated by this delay."
Mr Yousaf described it as "unacceptable" and "extremely disappointing".
He said: "Network Rail have failed to notice some foreseen circumstances.
"There is the potential there could be cost increases."
In a letter to the rural affairs and connectivity committee, Mr Yousaf wrote: “Network Rail has now advised that it expects the date for the first electric passenger services on the Edinburgh to Glasgow via Falkirk High line will be 2 October 2017."
"The programme to achieve this date includes a process of replacing the failed safety critical electrification component and a number of other critical works needed to complete the system to a satisfactory standard."
Electric services will be initially operated by Class 380 ScotRail trains switched from other lines, followed by brand-new Japanese-designed Hitachi Class 385 trains from December.
The minister wrote: “Whilst this delay is extremely disappointing, I am pushing Network Rail to maintain the December 2017 milestone – full electric service delivering faster journey times between Edinburgh and Glasgow via Falkirk High route with new, longer Class 385 electric trains.”
He added that the overhead lines not going live until September "has inevitably resulted in a knock on impact on the testing, approvals and driver training programme for the new ScotRail Alliance Class 385 trains.
"Network Rail has a critical role to play in expediting this rolling stock commissioning programme for the new trains, which must now be delivered in a compressed timescale."
The minister said Network Rail had drafted in 20 per cent more workers to make sure there were no further delays.