A property developer who illegally modernised the historic building behind the hymn All Things Bright and Beautiful has been told by a judge to pay out £300,000 or go to jail.

Grade II*-listed Llanwenarth House was built in the late 16th century and was given its special status six decades ago because of its national importance. The Georgian-style manor in the picturesque Usk valley, south Wales, is where Irish composer Cecil Alexander is thought to have written the lyrics to the famous song.

Newport crown court heard that Kim Gregory Davies carried out dozens of unlawful alterations – including the installation of a mosaic-clad Jacuzzi, ripping up parquet flooring and laying down modern tiles in their place as well as removing old timber windows. Prosecutors said Davies also wrecked several of Llanwenarth House’s Regency features and opted for a mock-Tudor design instead.

And the court heard the property appeared to have wall plaques inside that had been made from the gravestones of three children who died more than 100 years ago.

Davies, 60, originally insisted he had done nothing wrong and claimed planning officers had given him verbal permission to carry out the work. He also said he had saved the seven-bedroom property from ruin as it was falling down when he bought it.

But prosecutor Nicholas Haggan QC told the court of how listed building specialist Michael Davies described the alterations as being the worst example he had seen during his 25-year career.

He said: “The works carried out by this defendant were extensive and affected every part of the exterior and interior of the property. No application for any listed building consent was made – either before, during or after the works had been carried out.

“Most of what was originally in that property had been stripped out and dumped. Layers of history were ripped out and discarded. The impact of the works has been immensely damaging.”

Judge Daniel Williams ordered Davies, to pay a £60,000 fine by September 15. He will also have to pay £240,000 of the prosecution’s £440,000 bill.

Judge Williams said if the fine was not paid in full Davies would be the subject of a 20-month prison sentence.