A source on the project said it was “a miracle” no one died when pipes carrying concrete from sea level to an upper deck on the new Queensferry Crossing burst earlier this month.

The incident, which occured on June5, has set back progress on the bridge by around a fortnight and could cost hundreds of thousands of pounds to fix, but Transport Scotland insisted the project would still be delivered within budget.

The insider described how 30 cubic metres of concrete – about six lorry loads – were poured before it was realised the consistency was wrong.

Workers were later forced to dig out the mix after it had set, damaging steel rods used to strengthen the huge ­structure that will have to be replaced.

The problem was caused when barges of concrete heading for the bridge were held up by traffic on the water at the port of Rosyth.

The source told the Daily Record: “There was a delay transferring the concrete from the shore to the base of the tower. By the time it got there it had started to stiffen up.

“When they started to pump it up, some of the pipes exploded. It was sheer luck that no one was badly injured or killed.

“A few of the lads told them to stop the job because the concrete wasn’t correct but the bosses told them to press on regardless.

“They’d poured about six lorry loads of mix before they realised it wasn’t working and pulled the plug. When it set, it was useless – it was far too brittle.

“It will all need digging out by hand, which will take days. The delay will take at least two weeks to sort out before they can get going again and could cost hundreds of thousands.”

The new bridge is set to open to traffic next year, but buses, pedestrians and cyclists will continue to use the existing Forth Road Bridge.

A spokesman for Transport Scotland said: “Difficulties were encountered on the first concrete pour on the north tower deck last Friday, causing the pour to be stopped.

“The project has a set, unchanged budget range of £1.35bn to £1.4bn.”