Commuters will not be travelling on the new Japanese-designed fleet until “early next year” rather than December, ScotRail told The Scotsman.
The train operator blamed the latest setback to the troubled project on problems at train builders Hitachi’s factory in County Durham.
It comes after the project to provide faster and longer trains was put back twice because of difficulties with electrifying the line, which should have been completed last December, then July.
ScotRail Alliance managing director Alex Hynes said: “It was envisaged there would be a full electric service in December. That’s no longer looking likely.
“The Class 385s, which are being built by Hitachi, are behind programme, and that’s only emerged in the last few months. Clearly, we are very disappointed about that.
“It is the first time Hitachi have properly built a train in the UK. They are doing this for the first time and they have hit some teething issues.”
Mr Hynes said he did not want to jeopardise the line’s current good punctuality and passenger satisfaction by introducing the trains before they were fully tested.
However, Hitachi said testing had been delayed by access to the route being put back from March to next month.
Its spokesman said: “We still require access to infrastructure in Scotland to complete full and thorough testing.
“We’ve always been clear that once we get access to the newly electrified infrastructure, it will take a minimum of four weeks to test, followed by at least 12 weeks of approvals work before trains could safely come into service.
“Under the original March timetable this would have been sufficient.”
The trains were due to cut the 51-minute journey to 47 minutes in December and 42 minutes in December next year.
Mr Hynes said ScotRail’s other electric trains may operate some services on the route from December as a stop-gap.
Scottish Labour transport spokesman Neil Bibby said: “Passengers that travel between Glasgow and Edinburgh have had to put up with major disruption over the past few years and they were given a clear promise from ScotRail and the SNP Government that the new electric trains would be running this year.
”Passengers deserve an apology for another delay with the project and deserve answers as to why passengers have been let down yet again.”
Transport minister Humza Yousaf said: “It is deeply disappointing we won’t have all of the new trains in service this year as expected.”
But he added: “Rail industry experience on previous new train introductions across Britain demonstrates the advantages of a controlled and phased programme to best manage performance risks.
“Given the efforts made by ScotRail over the last 12 months to lift performance, it is understandable for them to say passengers would not want to risk an unnecessary dip in that performance.”