For the first time in over 20 years, the Government has proposed major changes to The Treasure Act 1996 in an effort to keep national treasures accessible to the public.

Currently, the law states that for an artefact to be declared as treasure, and therefore offered to a museum for first refusal, it must be substantially made of gold or silver and at least 300 years old. Items are also legally considered treasure if they are discovered alongside artefacts of precious metals. Finders are at present under no obligation to report their discoveries.

The new plans propose to expand the definition of treasure to include anything with a value over £10,000. By doing so, museums would be given first refusal on all valuable finds before they are openly sold to private buyers.

Michael Ellis, the heritage minister, commented: “these new proposals will help our museums acquire these treasures and make it harder for nationally important finds to be sold for profit.”

These welcome changes follow the case of The Crosby Garrett helmet, which was sold at Christies for £2.3m to an unknown private buyer in 2010. Unearthed in Cumbria by an amateur metal detectorist, the magnificent Roman helmet was made entirely of copper. Due to its material, the helmet did not fit the requirements to be legally defined as treasure. Its sale was described as “a real blow” by the local Tullie House museum, who unsuccessfully attempted to raise funds to purchase the helmet.

Michael Lewis, head of the portable antiquities scheme at the British Museum, noted that “in the case of the Crosby Garrett helmet, for instance, the finder did tell us about it, but ultimately it’s not in a museum collection – it’s in private hands. If you or I wanted to go and see it, we can’t, and that’s a loss to our national heritage.”

Since 1996, the number of items discovered by treasure hunters has increased by 1500% and a total of about 13,000 artefacts have been discovered in the last 20 years. Many of these nationally important finds were undeclared and sold to private collectors, with only 30% now displayed in museums.

The new changes to The Treasure Act 1996 are open for public consultation until 30 April.