Repair works on the Queensferry Crossing will continue to autumn 2019, two years after the flagship £1.34bn bridge opened.
Transport secretary Michael Mathieson said he is “disappointed” difficulties with resources has led to snagging repairs being delayed.
The works were due to be completed by September this year but Transport Scotland director Michelle Rennie gave MSPs a new timetable showing some issues have a target date of October 2019.
Jamie Greene, Scottish Conservative transport spokesman, said: “Commuters will see news of more repairs needed and conclude that the SNP is now playing catch-up after hurrying to open the bridge triumphantly, at the expense of commuters and drivers.
“The fact the bridge is architecturally impressive does not excuse any disruption that will be caused to drivers due to its premature opening.”
Mr Matheson refuted the claims and said: “Our contractor, Forth Crossing Bridge Constructors (FCBC) have indicated they have not made the progress with the snagging and finishing works as originally planned.
“The target date for completing this work was September 2018 but they have informed us they require further time.
“FCBC have said they have experienced difficulty in mobilising the resources required to complete the outstanding works on the project but have expressed that they are confident they will finish all but two areas of the remaining works by December 2018, with work on the remaining two areas to commence in spring 2019.”
He continued “It is a standard approach for major projects to open traffic whilst some works remain outstanding.”
The minister stressed that the bridge is providing a reliable replacement crossing over the Forth and the works still required can take place without closing it and there would be no further cost to the contract.
He added: “While I am disappointed with this news, Transport Scotland will continue to work with our contractors to ensure they finish at the earliest opportunity possible.
“When the work is ongoing we do not expect average journey times over the Queensferry Crossing to be impacted.
“Where possible traffic management will only be implemented during the night, as is standard across the trunk road network.”