Work on the £1.3 billion Queensferry Crossing has been halted for an average of two days a week since November – because of bad weather, mostly high winds.
Engineers behind the 1.7-mile structure – alongside the Forth Road Bridge – reckoned on losing 32 days to the weather in that period. But the actual total has been 71.5 days.
Separate figures show “high” winds were recorded on nearly half of the days in 2015 at the neighbouring Forth Road Bridge, with contractors last night under pressure to explain why the construction programme didn’t make more allowance for bad weather.
SNP ministers expect the bridge to open in May next year, five months later than previously stated.
Labour’s transport spokesman Neil Bibby said: “No government or contractor can control the weather but there should have been much better planning for all eventualities.
“It looks like this project had very optimistic expectations for Scottish weather that have unsurprisingly not been met.
“It’s little wonder that the opening date for the bridge has been delayed by at least six months.
“Given there have been more than double the number of days lost to bad weather than expected, SNP ministers must confirm what discussions they have had with Transport Scotland about this and whether there will be any additional cost to the taxpayer because of the delays.”
Work to erect the cables on the bridge is affected by winds of more than 25mph and the installation of decking units by speeds over 34mph.
Records from the Forth Road Bridge show there were 158 days last year when average wind speeds and gusts topped 30mph. The tally was 142 in 2014 and 131 the year before.
“The fact that we have seen double the number of bad weather days than were forecast raises questions over where these numbers came from,” said Alex Cole-Hamilton, Liberal Democrat MSP for Edinburgh West.
“The upshot of this failure of planning is that motorists are facing months more disruption and delays.”
A Transport Scotland spokesman said: “All that can be done has or is being done to achieve the earliest possible opening date to traffic but as we have stressed, no-one can control the weather.”