The theft took place when a stallholder, who does not wish to be named for security reasons, was clearing up at the end of the two-day fair between 3.30pm and 4pm and packing away jewellery, which he placed in a black holdall under his stall. He then went to his car, leaving his wife to mind the stall. On his return, he found the bag missing.
The stolen jewellery, which included rings, necklaces, earrings and diamonds, is valued at more than £80,000.
A spokesman for dmg Antiques Fairs said that the local police had responded very quickly, but neither they nor the onsite dmg staff were able to find the culprit.
Details of the stolen items are being passed by the fair organisers to police who specialise in antiques crime who are employed by dmg at the Newark Antiques Fair.
PC Russell of Haywards Heath Police Crime Team is the officer in charge of this case and can be contacted on 0845 6070999 quoting crime reference number NM1/5120/08.