Arthur Brand, dubbed the ‘Indiana Jones of the art world’, said he handed over the centuries-old carvings to the Spanish Embassy in London at a private ceremony.
It was the culmination of a long search for the artworks, which were snatched in 2004 from the Santa Maria de Lara church in northern Spain, believed to be at least 1,000 years old.
They turned up in the garden of an aristocratic British family who had unwittingly bought them, and it was there that Brand found them covered in mud and leaves.
"These artworks are priceless. To find them in a garden after searching for eight years is just incredible," the art sleuth told AFP.
The private investigator showed AFP the stone carvings – one of which depicts John the Evangelist, author of one of the Gospels – hours before they were handed back.
The reliefs, said to weigh 50 kilos each, were then handed to representatives of Spain's Guardia Civil police force, who are collaborating in the case, and two museum curators from the northern city of Burgos.
The church from which they were stolen is believed to date from the era of the Visigoths, with experts dating it between the seventh and eleventh centuries.
But in a huge blow the reliefs were snatched by professional art thieves in 2004, Brand said.
In 2010, the intrepid sleuth first received word from an unnamed British informant that "something strange" had popped up in London, which eventually pointed him to a shadowy man Brand referred to as 'Mr. X'.
"It turns out 'Mr. X' saw a French dealer arriving with the reliefs by truck in London. They were put on offer as garden ornaments – but Mr. X recognized them as possible Visigoth religious artworks," Brand said.
Brand then tracked down the French dealer, who pointed them towards an unnamed British aristocratic family living north of London.
"It ended up in the garden of an English nobleman, who did not know that it was world heritage, where they would stay like 15 years," he said.
The owners were so shocked when told the truth that "they wanted to throw the artworks into a river and let them disappear forever. Fortunately, we managed to convinced them not to," said Brand.
It was yet another success for Brand, who was in the headlines last year for returning a stolen 1,600-year-old mosaic to Cyprus that was found in the possession of a similarly unwitting British family.