Investigations into the cause are still continuing but information has come to light showing wind speeds before the accident exceeded the maximum safety limit.

The wind was 4mph over the restriction laid out in the contract

The four men who died - including 42-year-old Jeff WIlliams from Newport - were using an American-built gantry while carrying out repairs to strengthen the bridge that carries the M5 over the River Avon.

But at mid-day on 8 September it collapsed withiut warning, throwing the men 80 feet to their deaths.

But in the windy conditions of that afternoon the BBC understands that they should not have been working on a gantry. A contractual clause restricts the use of gantry platforms in adverse weather.

Jeff Williams from Newport was one of those who died

It says: "Use of gantries will be suspended when wind speeds exceed 12 metres per second."

This equates to 27mph but it is not clear whether this restriction relates to continuous wind or gusts of wind.

Meteorological Office records for the time of the accident show local winds were gusting south-west at up to 15 metres per second or 31mph - four miles over the restriction imposed by the contract.

The union alledges that at the time of the accident inadequate tethering allowed the gantry to move suddenly in a gust of wind.

The supporting trolleys ran off the the beam and the four unsecured men were thrown onto the South Bank of the River Avon.

Wind then carried the now dangling platform, some 800 metres across the river where it crashed into scaffolding.

None of the gantries are now in use and an independent investigation is being carried out into their design.

It's just one aspect the Health and Safety Executive is looking at - it too is studying the windspeeds at the time of the accident.

The Health and Safety Executive says its report into the accident should be completed by early next year.

'System is safe'

The aluminium gantry was installed by the contractors on the Avonmouth Bridge in 1996. It was one of fifteen being used when the accident happened.

Experts say this system is robust and safe if all the guidelines for its use are carried out.

The main contractors Costain and sub-contractor Kvaerner Cleveland Bridge said they were "unable to make any comments until investigations into the accident were completed".

In the meantime work continues on the bridge. It has been plagued with problems since efforts to strengthen it began four years ago.