The opening of the Queensferry Crossing has been postponed from May until between mid-July and the end of August, economy secretary Keith Brown told MSPs today.
He told the rural economy and connectivity (REC) committee the planned end of May date was "not now safely achievable", which was a "huge disappointment".
The new setback means the bridge could open eight months late.
It follows the opening being put back last June from December to May because of strong winds hampering construction progress on the £1.35 billion bridge.
Mid Scotland and Fife Conservative MSP Murdo Fraser, who questioned Mr Brown on the issue in the Scottish Parliament yesterday, tweeted: "News that Queensferry Crossing might not now open until August will be met with anger & dismay from my constituents.
"Twice in last few months, they have suffered massive disruption due to closures of existing Forth Road Bridge.
"Lots of Qs now about new delay & when Scot Gov knew."
Mr Brown said the latest problems included strong winds delaying the dismantling of the tower cranes.
Other work in the vicinity also has to be halted during the operation.
Mr Brown said the first of the three cranes had taken 65 days to take down rather than an expected 15 days.
He said health and safety must be the top priority, especially since the death of a worker last year.
However, he stressed the cost of the over-run would be borne by the contractors, with no extra cost to taxpayers.
The construction consortium is also liable to pay penalties if they breach the end of the contract date in mid-June.
Mr Brown had told MSPs yesterday that "adverse" weather had delayed the opening for a second time – despite extra time being added when it was last postponed.
The original delay came despite a year of contingency time being built into the five-year construction schedule for the Forth Road Bridge replacement.
Last June, Mr Brown assured MSPs that the Forth Crossing Building Constructors consortium (FCBC) had “built-in allowances for future weather based on the experience to date”.
However, FCBC board chair Sally Cox told the committee today there had been 175 per cent increase in time lost to bad weather in January than had been included in those allowances.
Mr Brown said: "There are only nine weeks to the end of May. Even with the best weather, this date is not now safely achievable".
Today's announcement came after Mr Brown considered overnight a report from officials following an update on Monday from FCBC to the Scottish Government’s Transport Scotland agency, which is in charge of the project.
This had been compiled by experts brought in from across the world, Mr Brown said.
A Transport Scotland spokesman said today: "The contractor has reported there have been fewer clear weather windows than expected, particularly in relation to wind.
"This has delayed weather-dependent activities, causing them to bunch together at the end of the programme to a much greater degree than was anticipated in May 2016.
"This means a lot of construction activity is underway simultaneously, and careful planning is required to ensure that this can be done safely, so that one activity does not impact on another.
"At this stage of the project, all remaining activities are vulnerable to weather conditions.
"For example, the tower cranes can only be removed in wind speeds of less than 25 mph, and when this is happening, it is not possible to safely work within a 50-metre radius of the crane.
"Other works are also affected. For example, work on cable stays is sensitive to wind, waterproofing the bridge deck is sensitive to rain and road surfacing is sensitive to both rain and low temperatures."
Mr Brown said: "While its clearly very disappointing the new bridge won’t be ready ahead of the contractual completion date, I believe it is important to take the time to make sure the bridge is built in the safest possible circumstances to the highest possible standards.”
The delays have led to 200 workers being drafted in, with the workforce increasing to more than 1,500 in February.
Scottish Liberal Democrat transport spokesman Mike Rumbles said: “Workers made it clear to me in October that this project was not going to be delivered on time and that the SNP Government had not built in enough contingency time.
"Did the economy secretary seriously not suspect anything until Monday?
“It is now eight months behind the timetable heralded just last May.
"For a project that was supposed to showcase the competence of this Scottish Government, there has been an awful lot of rescheduling and there is still a lot of work to be done before the crossing is finished.”
Scottish Conservative transport spokesman Liam Kerr said: “This delay will come as a huge blow to the many commuters who travel across the Forth every day.
“They have already had to deal with massive disruption over the past two months due to closures on the Forth Road Bridge, and now we learn the Queensferry Crossing is months behind schedule.
“The SNP assured us this project would be delivered on time, but these promises have proven to be worthless as, once again, we see the completion date slipping.
“It is simply unacceptable, and the economy secretary needs to explain what the Scottish Government are going to do to ensure the new timescale is kept to.
“There are also underlying questions about how long Keith Brown has known about this issue.
"Many contractors had been hinting at delays in the previous few weeks, so if they knew it was going to be delayed, why didn’t the minister?
“It points to someone who is either hopelessly out of touch with this project, or someone who was waiting for a busy news day in order to bury bad news.”