By comparison, there are 50,000 official tickets still on sale of the 2.5 million total, with more to be released soon.
Charlie Abrahams, senior vice president at the firm, said people it was almost impossible for buyers to tell if the tickets were simply being resold by fans who could no longer attend, or were counterfeits. Some might not exist at all, he said.
But even if fans do get hold of genuine tickets, the Rugby World Cup organisers are threatening to refuse entry if the seats were resold on unofficial websites.
"We were surprised by the scale of the unauthorised tickets being offered online," Mr Abrahams said. "The authorities are clearly having difficulty controlling these sales because the source is often abroad and cannot be tracked down."
Fans are allowed to resell tickets only through the official event portal at tickets.rugbyworldcup.com. All tickets listed on other reseller website may be seized or cancelled without refund or compensation, according to the tournament organisers.
People who buy from an unofficial seller risk being turned away at the ground.
Trading Standards is urging fans to check websites are official sellers before buying, using a tool at rugbyworldcup.com/buyofficial.
Mike Andrews, of the National Trading Standards eCrime team, said: "We are warning fans not to succumb to last-minute Rugby World Cup fever and fall foul of scammers and rogue ticket agents who will be looking to cash in on their enthusiasm.
"People need to understand that only those fans with tickets bought from official sources will be guaranteed admission to matches, and tickets not bought from official channels are liable for cancellation, so heeding our advice could help avoid bitter and costly disappointment at the turnstiles."
Police have already suspended a phone line selling tickets for next month's Rugby World Cup.
Which, the consumer group, found GetSporting.com was offering tickets for sold-out matches such as England v Australia for £500 and for the final at almost £1,800.
