A footprint supposedly cut in stone around 1500 years ago to mark the birth of the Scottish nation is not the real thing, Historic Scotland has admitted.

The royal inauguration stone at Dunadd, in Argyll, where an estimated 50,000 visitors a year place their feet in what they believe to be the actual ceremonial footprint used to initiate the earliest Scottish kings, is in fact a fibreglass copy of the original, placed at the site in the 1980s.

The real stone remains where it has always been, on an exposed outcrop in the bottom of the Kilmartin Valley. But it is entirely hidden from view, buried underneath its synthetic counterpart, which has been integrated into the surrounding turf to make it look as authentic as possible.

The replica is presented as being the genuine article in order to discourage visitors to the site - which is managed by Historic Scotland but is unmanned and has no visitor centre - from trying to remove it and touch the original below.

Chris McGregor, Historic Scotland conservation regional architect, told the Sunday Herald: "What we've got here is of great significance. The real king's footprint is on-site, but we have protected it with a layer of synthetic material.

"Generally, we don't tell people that it is a replica because of the risk to the original if people try to rip it up. We interpret the site as though the replica were the original."

So convincing is the copy, made using a cast that mimics the original stone's early Celtic runes, carving of a boar and the footprint, it has tricked even academic experts into believing it is genuine.

Dr Dauvit Broun, a history lecturer at Glasgow University and a specialist on early mediaeval Scotland, admitted he had been fooled by the replica. But he said Historic Scotland had come up with an "ingenious" way of balancing access against conservation.

"Historic Scotland has got a problem," said Broun. "There is this footprint, which is very precious, and they know thousands of people will come and put their foot in it. But that will also damage it. Other organisations might have ringed it off, but by doing this it allowed people to get what they want.

"It is interesting that people are happy with a lot of Scottish myths and legends but might get upset about something like this," added Broun. He pointed out that the practice of replicating original historic artefacts was commonplace in other countries, such as Japan.

However, Broun said the question of whether or not the visiting public should be made aware that the "stone" they can see and touch at Dunadd is not the real thing deserved discussion.

"This is a unique situation because Historic Scotland can't move the original," he said. "But what are the alternatives? Do you encase it with a building? Personally, that would distress me. It would distort the impression you get of the site. Or do you box it in, as has been done with some Pictish stones? I think that would be wrong."

McGregor said there was no intention on the part of Historic Scotland to deceive the public by faking the stone. "The decision to construct the replica was made at the time with the best of intentions," he said. "We have done this sort of thing in a number of locations."

The inauguration stone at Dunadd, not to be confused with the Stone of Scone, is one of the most important heritage sites in Scotland. Dunadd was once the capital of Dal Riata, the early kingdom of the Scots that was established in Argyll around 500AD.

In a semi-pagan ceremony, akin to the mythical Arthur pulling the sword Excalibur from its sacred rock, would-be Scottish kings had to place one foot in the stone footprint to activate their kingship. This act symbolised the king's marriage to the land, energising the soil so that harvests would be plentiful and the people successful in war. Kings believed to have been inaugurated at Dunadd include Kenneth Mac Alpin, who in 843 united the kingdoms of Dal Riata and Pictland.

It is estimated that around 50,000 visitors go to Dunadd each year. "That's a lot of footprints and potentially a lot of damage," McGregor said. "So the replica is inspected from time to time to make sure it is in good condition.

"Sometimes members of the public report to us that the stone' is damaged. We then send our conservation team out to repair it and keep it looking as authentic as possible."