ScotRail's new fleet of electric trains will not enter service on the main Edinburgh-Glasgow line until late March, The Scotsman has learned.
The Japanese-designed Hitachi Class 385 trains were due to have been introduced on the route in September.
However, there have been a series of problems with electrifying the line and ScotRail said there had also been setbacks with train production at Hitachi's factory in County Durham.
An industry source said the target for passenger service was now late March - six months late.
They were also due to have started operating on the Edinburgh to North Berwick and Dunbar lines from next month.
The news came after ScotRail Alliance managing director Alex Hynes told MSPs that sufficient trains to operate the service were not expected to be delivered until February.
But he said the trains and newly-electrified line both had to be tested.
He told Holyrood's rural affairs and connectivity committee: "I'm not yet prepared to make a commitment [over introduction] because of the complexity of what we are trying to achieve."
The Conservatives said the delay was "hugely disappointing" to commuters.
Mr Hynes said: "I can't give you a cast-iron guarantee because it's not a process I am wholly responsible for" - since Hitachi and regulator the Office of Rail and Road are also involved.
He said he expected to have 21 of the trains available in February, but repeated his view that "rushing them into traffic" before being fully tested could hit the route's performance, which was "the last thing customers want".
They will run with up to eight carriages compared to the current maximum six, and reduce journeys by ten minutes to 42 minutes, from December next year.
However, Mr Hynes said there were no plans to run a non-stop 30-minute service between the two cities.
He said: "The correct balance between capacity and journey time is probably right, but we will keep it under review."
Mr Hynes said other electric trains would start running on the Edinburgh-Glasgow main line from next month as planned - although that is a year later than scheduled.
The spare Class 380 trains - which operate on lines such as Glasgow-Ayr and Edinburgh-North Berwick - will take over some peak-hour services from early December.
Scottish Conservatives transport spokesman Jamie Greene, who sits on the committee, said: "Whilst we accept some of the testing functions are outside of the control of ScotRail, and testing is an essential part of implementation, there still seems to be no guarantee as to when the Class 385s will be fully running on this busy and important line.
"This is hugely disappointing news for Central Belt rail commuters.
“There seems to be a constant blame game between Hitachi, ScotRail, and now these “external factors” which will affect testing and implementation.
"Humza Yousaf is the transport minister and he really needs to get a grip over delivery of these long overdue trains."
Scottish Labour transport spokesman Neil Bibby said: “The SNP Government and ScotRail gave a clear promise to passengers that these new trains would be operational months ago.
"Passengers travelling between Glasgow and Edinburgh have had to put up with delay after delay and it is unacceptable.
"Scotrail have told Parliament the new trains may be fully operational by February but have failed to give a clear guarantee that they will be in service by then.
"Passengers will again be concerned - not giving a guarantee is very telling and simply not good enough."