For years, the antique had been on a rickety support on a sideboard of Wendy Jones's home without anyone suspecting it was of such high value.

Mrs Jones only took it to the show because her husband was taking some books.

But she was startled when the 22-inch oval-shaped plate turned out to be the most valuable to have appeared on the BBC programme in its 30-year history.

The plate was commissioned by the Prussian East India Company for King Frederick II, who set up the company in 1750.

It was made from hard paste porcelain and decorated with the alms of the Hohenzollern family, the order of the black eagle, and the Maltese Cross.

The plate was made between 1750 and 1755, and there are pieces of the service in museums across the world.

Mrs Jones said: "The plate actually belongs to my son because he was left it by his grandmother.

"He didn't have room for it in his London home so I had it.

"One day it fell off the stand and crashed onto the sideboard but luckily it wasn't damaged.”

As she left the house to go to the show in Aberglasney, Wales, she grabbed the plate and put it in a single Tesco's carrier bag. They can easily split,” she said.

"When I heard how much it might be worth I was shocked.

"On the way to the show the plate was on the back seat of the car, but on the way home I kept hold of it all the way.”

She added: "We are not sure where the plate came from but my son's paternal grandmother did marry into a German family.

"And the plate was made for the King of Prussia, so that's possibly how it ended up in our family.”

John Axford, from Woolley and Wallis auction house in Salisbury, Wilts, who valued the plate, said: "It is a fantastic piece of a very rare Royal service.”