A huge block of stone flew out into the road when Temple Mill partially collapsed.

But the true extent of the damage is only visible once you are inside the historic treasure that once had sheep grazing on its grass roof.

Two sections of the vaulted ceiling in the grade I listed building on Marshall Street, Holbeck, collapsed in December, forcing one of its 18 ornate pillars to crumble and prompting panic over the former mill's future.

Now a major seven-week programme, costing around £230,000, is under way to make the structure safe.

An intricate 'bird cage' scaffolding system – stretching the width of the building and all the way to the roof – is being installed to add strength to the columns, prevent further damage and protect workers.

Because of the risk of collapse and falling masonry, engineers have to erect the scaffold from beneath a mobile shield more commonly seen in the mining industry or tunnelling works.

The temporary structure will hold the columns in place until a permanent version can be erected, once planning permission is secured.

Site manager Stewart Kellett, facilities manager for owner SJS Property Management, said: "The fact that it collapsed and managed to contain it as it did is testament to the way it was built all those years ago."

He added: "There's actually very little, considering the extent of the damage inside, that came out on to the road."

The mill and neighbouring offices were built for John Marshall, founder of the Leeds flax industry, around 170 years ago.

Architect Joseph Bonomi modelled the buildings on the Temple of Horus at Edfu on the banks of the River Nile.

In the long term SJS plans to turn the building into a centre for culture and specialist retail.