The four men from Suffolk were building a test facility at Claxton Engineering in Great Yarmouth in 2011.

The BBC said that an HSE investigator told the coroner’s court in Norwich that the steelwork collapsed “like a folding picnic table”.

The inquest, which is being heard by a jury, is due to last five days.

Daniel Hazelton, 30, his 26-year-old brother Thomas Hazelton, Adam Taylor, 28, and Peter Johnson, 42, died on 21 January 2011.

The men were working in a 7ft deep hole which measured 75ft x 10ft across, the inquest at the Sprowston Manor Hotel in Norwich heard.

A steel reinforcing cage, comprising of 39 tonnes of steel and 330 tonnes of concrete, was being erected inside it.

Jonathan Elvin, HSE investigator, told the court the excavated hole was wet and the men were reinforcing sheet pilings when the steel toppled “collapsing like a folding picnic table”.

Post-mortem examinations revealed they all died of asphyxia due to trauma.

The men were working for Hazegood Construction, of which Daniel Hazelton was a director.

Mark Aylen, procurement manager for Claxton, told the court he had seen the men not wearing safety helmets and using grinders and other equipment without wearing safety glasses.

He said when he questioned them about this, they told him hard hats fell off and that safety glasses steamed up.

The inquest continues.