The news follows drivers over the River Forth suffering months of delays from snagging work, which was due to be finished in September.
However, new transport secretary Michael Matheson has revealed that painting the underside of the road deck will not now be finished until December 2019.
This is because of delays installing a platform to ensure the safety of workers in all weathers.
It means the finishing touches will not be put to the bridge until more than two years after it opened last August.
Last November, it was revealed that off-peak lane closures would continue for ten months to finish the project.
But Mr Matheson said the painting work would not affect drivers.
He said contractors building the £1.35 billion crossing had “confirmed that undertaking this work will not compromise the use or integrity of the structure and traffic flow will not be impacted”.
In a letter to Holyrood’s rural economy and connectivity committee, he said: “Whilst Forth Crossing Bridge Constructors (FCBC) have provided their firm assurance that the work will be completed, they have requested a relaxation of the delivery timescale to deliver the work in the most economical manner possible, allowing them to better manage available resources. “
“In this context, and in the knowledge the work will be completed at no additional cost to the public purse,
“I consider it appropriate that additional time is permitted to allow the contractor to work in the most economical fashion and to complete the works by the end of 2019.”
Mr Matheson said his officials had been assured by FCBC they were on schedule to complete the rest of the 23-point snagging and finishing list by September. It includes lighting, drainage and concrete work.
Last November, one carriageway was closed for five days for repairs after surfacing around expansion joints was laid too high Transport Scotland had said the snagging work would have a “minimal impact” on traffic.
However, drivers have complained that even late evening Edinburgh-Fife trips have taken twice as long as normal because of lane closures.
John Baxter tweeted: “What’s going on with Queensferry Crossing? It’s back to looking like a building site. What a mess and many delays.”
Peter C tweeted: “10pm. Long queue when you just want to get home! Finish this bridge!”
Neil Greig, of motoring group IAM RoadSmart, said: “Despite assurances on traffic flow being unaffected, any activity around the bridge could cause delay due to distraction or extra works vehicle movement.
Although it is absolutely essential the builders get everything right, these ongoing uncertainties are not good for people and businesses trying to plan ahead.
“Traffic management around the bridge also needs to be constantly reviewed to maximise the benefits of the massive investment in this new iconic structure.
“So far, a few extra days of unlimited operation during high winds seems little reward for drivers who still find themselves in long queues every morning and evening.”
Scottish Labour transport spokesman Colin Smyth said: “The Scottish Government have consistently claimed the crossing would be completed by September, so it is hugely frustrating this work will drag on for another year.
“The SNP are quick to pat themselves on the back when it comes to the crossing, but they’ve been slow admitting to the yet more delays and the failure of the contract to deliver on time.”
His Scottish Conservatives counterpart Jamie Greene said: “In its first year, the bridge has been subject to constant congestion and lane closures as contractors perform vital work that should have already been completed.
“Minor works on infra- structure projects are expected, but given the time it is taking to complete them, it raises questions about whether the bridge was opened too early.”
A Transport Scotland spokesman said: “It was always our intention to open the bridge at the earliest opportunity to ensure the benefits of a more reliable crossing over the Forth could be realised quickly.
“FCBC has informed us they intend to complete all outstanding work, apart from under deck painting, by the end of September.
“All of this work forms part of the original contract and there is no additional cost to the public purse.”