Support structures for the new Queensferry Crossing will not be completed until "the summer of next year", Transport Scotland has said, throwing further doubt on claims the £1.4 billion bridge project can be completed by its December 2016 deadline.

Thomas Nilsson, the construction director of the international consortium Forth Crossing Bridge Constructors, said in March last year that of the nine V-shaped piers and three 210-metre towers: "If everything goes OK, we are going to see the piers and towers ... going up this summer [2013]".

However, this target date is set to be missed by as much as two years, with three of the 50m piers (known as S1, S2 and S3) on the south side and one on the north side (N1) awaiting the completion of foundation work at or near sea level. Another (S4) is currently only half-complete.

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Transport Scotland says the south- side piers will be completed and ready to support the viaduct leading on to the bridge deck "by the end of this year", and equivalent work on the north approach "by summer next year [2015]".

The delay suggests to civil engineering experts that the consortium has experienced similar problems setting the pier foundations as thosew that affected the placing of the giant steel "caisson" base of the south tower, an operation that took over a year longer than planned.

Transport Scotland's project director David Climie last year admitted that previous delays had depleted the project's "float time" - time built into the construction's "critical path" to absorb contingencies.

The Sunday Herald has previously reported doubts about the consortium's ability to make up lost time, given the unprecedented length, height, architecture and engineering challenges of the new bridge, and the works programme's vulnerability to weather conditions in the Firth of Forth.

The reports were dismissed by Transport Secretary Keith Brown MSP, who said they were "based on ill-informed speculation and a lack of understanding."

A spokesman for Transport Scotland said: "The project remains on schedule to be complete by the end of 2016".

Asked if any perceived risk of delay would immediately be made public, he said "yes".