The British Hallmarking Council said it was concerned that consumers were being "duped" after a 10-day internet sweep found that 36% of gold jewellery listings - or 6,377 - had not been advertised as hallmarked.
Of those listings, 24% (4,278 items) were suspected as fake and therefore being sold illegally, it said.
The research suggests around 150,000 items of fake gold jewellery could be listed for sale in the UK each year.
British Hallmarking Council chairman Noel Hunter said: "The UK Hallmarking Act (1973) was put in place to protect consumers and retail jewellers from counterfeits, but the application of the legislation to online trading activity remains untested.
"And we have seen little appetite from the internet giants to step up enforcement or adequately protect consumers.
"Our internet sweep highlights just a fraction of the infringements made by online sellers of 'precious metal jewellery' in the UK today.
"A duty of enforcement currently rests with local Trading Standards departments, who have suffered a 50% cut in their resources over the last five years.
"Adequate powers are necessary to deal with internet trade."
Robert Organ, deputy warden for the Goldsmiths' Company Assay Office, said: "We are joining the British Hallmarking Council in calling on the Government to work with us and the other Assay Offices in the UK to develop a robust enforcement strategy that protects consumers and businesses from internet based, unfair trading practices.
"This must include a review of the current Hallmarking Act to see if it could be extended to cover internet trade.
"We are also asking the Government to work with Amazon and eBay to increase hallmarking information on precious metal jewellery listings, raising consumer and seller awareness about hallmarking and the law."
Mr Organ said consumers should always ask an online seller if an item has been hallmarked before they buy.
Anyone who believes they have been sold fake precious metal items should speak to Citizens Advice, who can provide general information on hallmarking or give advice to help resolve a dispute.