It is understood the delay is due to the time it is taking to finish work on the bridge’s road deck, with poor weather over recent months being blamed for the slow-down.
Completion of work on the structure is not now expected until well into next year.
One staff source said: “Everyone agrees there is no chance of it finishing by the end of the year. It could be months late - maybe even the middle of next year.”
Critics have described reports of the delay as deeply worrying.
Alex Johnstone, transport spokesman for the Scottish Conservatives, said: “Given the recent closure of the existing bridge, any delay will be a serious concern to commuters. The SNP needs to be open with them so everyone has a clear picture of what is going on.”
Days before last month’s Scottish election, SNP ministers insisted that the project remained “on schedule” to be complete by the end of this year.
A Transport Scotland spokesman said: “As previously stated, the contractor, Forth Crossing Bridge Constructors, is making every effort to open the bridge as soon as possible, despite the challenge that the weather presents.”