Project director David Climie warned MSPs that bad weather could still make it “challenging” to hit the target and accepted there were “certain circumstances where it could be after May” though he added that “everything that can safely be done is being done”.

The contract for completion of the bridge runs until June 2017, but it was originally hoped the crossing could open by December 2016. This deadline was revised when high winds forced work to stop for 25 days in April and May.

The project also remains within its budget of £1.35bn.

Mr Climie said that while changing the date was a “painful process”, the new target date was “realistic”.

He said there had been a “worse than average winter”, which had caused delays, but that he was now confident it could be open by May 2017 after a “good three months”.

Questioned by MSPs at Holyrood’s rural economy and connectivity committee, Mr Climie said: “I’m always an optimist so I hope it will be before May, but I’m also a realist so I accept there are certain circumstances where it could be after May.

“The main issue that could affect it is one we can’t control, the weather. May is a very reasonable assumption and that’s certainly what were aiming for, that we believe we can achieve and that the contractor is telling us they believe they can achieve.”

MSPs were told that 93 of 110 deck units were now in place, and that work had progressed well on supporting roads and infrastructure.

The committee heard there were still “incredibly frustrating” days when work could not be done due to wind, but that contractors Forth Crossing Bridge Constructors had shown flexibility to keep the project moving forward.

There was also a pause in the project after a 60-year-old worker was killed on the site.

Mr Climie said the fatal accident had been “a great shock to the project and a great setback to us all”, and said that safety was top priority for the project team.

The Health and Safety Executive are still investigating the incident, which occurred during “routine maintenance”.

Mr Climie was also questioned about why the completion date had changed by six months after previous hopes the project could open early.

He said: “It was reasonable at the beginning of March for the contactor to expect that the weather would improve, that we would get into a better period of weather, and that even though they were behind where they wanted to be, they could recover to get there by December.

“What happened in April and May was exactly the contrary of that, it was significantly worse than expected. And therefore rather than recovering time they were losing time.

“And realistically it comes to a point where if you’re trying to recover one month in 10, that’s realistically possible in terms of a major construction project.

“But if you’re trying to recover two months in seven, that’s really not realistic anymore and we have to say right, we’ve given this our best shot, we’ve thrown everything at it we can and we have to say sorry, we just aren’t going to get there.

“They key is that, if you don’t get December, that pushes you significantly into 2017. There’s a multiplying effect - a day lost in April or May is not equivalent to a day in January or February.”