Forth Crossing Bridge Constructors (FCBC), the consortium building the bridge, informed ministers that ongoing effects of weather on construction mean it will require more time to complete the bridge.

Keith Brown said the project would be finished within the contractual completion date of June 2017- six months after the “target” opening date of December.

Transport Scotland said in a statement: “Since September 2015 the downtime due to adverse weather, specifically wind, has been 40 per cent compared to the 25 per cent anticipated by the contractor. Until May, FCBC believed that they could mitigate these effects - however, the impact of the weather in April and May with 13 days and 12 days lost to weather was such that they have advised that they can no longer deliver the December 2016 target.

“Whilst FCBC have been able to mitigate the impacts of weather by increasing resources and running a number of activities in parallel they are now entering a stage of bridge construction which is technically very complex.

“In order to mitigate the on-going weather impacts that have arisen over the past few months FCBC has procured additional physical resource, increased staffing by taking on an additional 100 workers, increased working hours, altered construction methodologies where possible and challenged critical construction sequences to identify where any programme efficiencies could be found.

“They have now reached the stage where further additional resources will not bring the delivery date forward due to the complex technical nature of the construction work. In addition, the remaining construction activities can only be carried out sequentially, further limiting the ability to make further gains.

“Specifically the bridge deck and the cable installation process which began in September 2015 is particularly sensitive to wind and this increases as the cables used become longer and are installed at a greater height .

“FCBC have confirmed this creates an unavoidable knock-on effect for subsequent activities, road surfacing and wind barriers which will now take place in wet and cold conditions during autumn and winter 2016/2017.

“This is a project with its own unique weather challenges and the contractor has incorporated their experience to date with weather in their planning process. As a result, they now expect to open the bridge by May 2017, ahead of the contractual completion date.”

Infrastructure secretary Keith Brown said: “We will continue to work closely with the contractors and I will personally ensure that every pressure and every resource is brought to bear to deliver or even better the revised target date of May 2017

“Going forward, in order to ensure that this project remains on track, I have implemented enhanced governance procedures from Transport Scotland senior management and will receive twice weekly updates from the project team.

“We have always been ambitious about this project and have always worked towards a deliberately ambitious target. However, It is important to recognise that FCBC still fully expects the project to complete within the timeframe of their contract. This project is not late and there will be no impact on the public purse.

“The December 2016 target date was set to address concerns about the long term condition of the Forth Road bridge where it was originally believed that it would be restricted as early as 2017. These concerns have proven to be less immediate and the recently installed structural health monitoring system is providing assurance on the ability of the FRB to sustain traffic. However that hasn’t decreased our determination to complete this once in a generation project at the earliest opportunity.

“The Queensferry Crossing directly employs over 1,200 people, many of whom have been performing some of the most complex civil engineering ever seen in Scotland, in the highly challenging environment of the Firth of Forth. Over 12 million man hours have gone into the project so far and we should not lose sight of their hard work and dedication. Anyone who looks at the works in the Forth cannot fail to be impressed with their achievements to date.

“It is important to remember that in the space of around nine years remarkable progress has been made in advancing this project from feasibility study to near completion. By the end of 2016 the project will be largely complete with the approach roads on both sides ready for traffic and a continuous structure spanning the Forth. - and by the middle of next year, traffic will be flowing across it. The Scottish Government will continue to ensure that this iconic structure brings benefits to the people of Scotland at the earliest possible opportunity.”

The setback has come despite a one-year contingency being included in the five-year construction project to allow for bad weather.

Work on the bridge was also halted in April when a 60-year-old worker was killed on the site.

His statement came in a written reply to a Parliamentary question from Scottish Conservative finance spokesperson Murdo Fraser.

Mr Fraser said: “This is very disappointing news for people on both sides of the bridge.

“They were continually told by the Scottish Government that this project was ahead of schedule and would soon be up and running.

“Now we find this is not the case, so the SNP must now give an exact date when this will be completed by.

“We saw the sheer disruption caused to people and businesses when the existing Forth Road Bridge was closed over Christmas.

“It’s essential we know whether or not the patch-and-mend approach taken at that point will be enough to ensure it can cope until the Queensferry Crossing opens.”