“These huge sapphires are surrounded by a halo of diamonds and linked together by additional single stone diamonds, which are between 0.8ct and 1.5ct.

“When Queen Elizabeth altered the necklace, she removed four of the sapphire clusters from the back and remodelled and reset one of larger stones from the front, to create a beautiful pendant for the necklace, which looks to be around a staggering 30cts.

“The necklace now contains around 200cts of sapphires and 100cts of diamonds, which means it’s likely to be worth around £3.5million.

“The original piece, before it was altered, would most likely have been worth approximately £500,000 more,” they revealed.