It emerged at the weekend that the electrification of the line between the two cities would not be up-and-running until the autumn, almost a year behind schedule.
Now transport minister Humza Yousaf has admitted he was informed of the delay on May 25, a fortnight before the country went to the polls.
He told Scottish Conservative transport spokesman Liam Kerr that he received the letter from Network Rail chief executive Mark Carne more than two weeks ago, and will now come under pressure to publish the correspondence.
It is the latest disruption to hit Scotland’s railway services in recent years.
And while Mr Yousaf has tried to shift the blame onto Network Rail, he previously took credit for the project, telling Holyrood: “We are in the midst of one of the most significant transformations of our railway infrastructure in recent memory.”
Today’s rural economy and connectivity committee is expected to hear more about the delay to the scheme, which is part of a £795 million improvement programme.
Despite being reluctant to reveal details of the delays ahead of the General Election, the Scottish Government made several separate funding announcements during that period, prompting accusations of breaching rules on purdah.
Scottish Conservative transport spokesman Liam Kerr said:
“There are serious questions to be asked about why the Scottish Government kept this important information to itself for so long.
“The transport minister admitted he was informed on May 25, yet we had to wait until Sunday until those key details emerged.
“Many will suspect the impending General Election may have influenced the SNP’s decision to keep this bad news under wraps.
“That’s irresponsible government, especially during a period where the SNP was more than happy to make contentious funding announcements during a period of purdah.
“Now the minister has confirmed he has this letter, he should make it public immediately.”
Scottish Conservative MSP Jamie Greene, a member of the rural economy and connectivity committee, said:
“It is extremely concerning to hear the electrification of the line is yet again delayed, bringing us nearly a year over the Scottish Government’s original deadline.
“Passengers who have already had to put up with major disruptions to the line will be dismayed to learn that electric trains will not be operating until October.
“This is one of Scotland’s flagship lines connecting two of our most important cities – it needs to be fit for the 21st century.”