Delayed work on the deck of the £1.4 billion bridge is not scheduled to be finished until September.
It then needs fitting out – and official documents, obtained by this newspaper, show the work could take up to 19 weeks.
This timescale would take the project into next year and past the proposed December 2016 opening date.
The development comes in the wake of our revelation that construction work on other parts of the 1.7 mile crossing is months behind schedule.
Transport Scotland last night insisted there was no doubt over the bridge opening by the end of the year.
But Fife MSP and Lib Dem leader Willie Rennie described the latest disclosures as “alarming”.
He said: “We need an urgent statement from the transport minister to bring some clarity to the state of the construction process.”
Meinolf Droste, the engineer in charge of building the towers and road deck, said in May 2014 that “cable installation and deck erection will commence in earnest early next year”.
But work on the deck didn’t get under way until September 7 due to poor weather.
Project director David Climie explained to MSPs last year: “We had hoped to get going in the summer with the deck lifting but summer didn’t arrive”.
He then added: “We are still allowing a year for the [deck] operations to take place”.
And Juan Jose Consuegra Perez, the approach viaducts manager, also said the main cable-stayed deck sections are expected to be completed by this autumn.
Following completion of the deck, the bridge needs to be fitted out for electrics such as traffic monitoring and lighting.
Utility pipes also need to be installed as does a service monorail underneath the deck to carry workers and materials.
Using freedom of information laws, our reporter asked Transport Scotland how long this work would take.
It said road surfacing would “take a maximum 11 weeks to complete based on the planned current levels of resources”.
In addition, the mechanical and electrical installation, parts of which are already under way, would take a “maximum of 19 weeks” once the full deck is in place.
Both estimates were based on current levels of resources remaining in place.
While the works will be done simultaneously, if it did take the full 19 weeks from September, then the bridge will not be completed until January 2017.
And this is before factoring in other potential delays such as the weather as Mr Climie has warned the project needs “an average winter to remain on track.”
Concerns have previously been raised about delays to the construction of the bridge’s foundations and steel viaducts.
Labour deputy leader Alex Rowley said: “If the SNP Government is going to miss their deadline for the opening of the Queensferry Crossing they need to be honest with the people of Scotland now. There shouldn’t be any shortcuts here.”
A Transport Scotland spokesman responded: “A project of the size and scale of the Forth Replacement Crossing has a changing and wide ranging programme of works – with over 20,000 construction activities and over 1,200 employees.
“For complex projects of this nature, it is important to maintain some degree of flexibility in how these resources are used to take account of factors as such weather, ground conditions and availability of materials for example.
“If any one part of the project requires additional resources these are used in order to keep the programme on track.
“Deck lifting began in September 2015 and will take approximately 12 months, the deck fitting-out could take a maximum of 19 weeks, based on current levels of resource.
“There is an opportunity to increase this resource if required, to open the bridge to traffic in December.”
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