According to a new study released Thursday, the amount of money wagered on the black market on gambling websites has doubled to £ 2.8 billion in the last year.
According to an analysis done by PwC for gambling companies, around 460,000 people use unlicensed betting websites, up from 210,000 two years ago.
Gambling industry leaders say the growth "shows how the unsafe, unregulated black market is a growing threat to UK gamblers".
However, PwC's results in recent years have not been undisputed.
Last month, Neil McArthur, executive director of the Gambling Commission regulator, said PwC's 2019 black market results "do not match the picture of intelligence" due to no distinction between black market locations or automated systems or bots.
However, Michael Dugher, CEO of the Betting and Gambling Council, said the 2020 report was "an impressive and comprehensive piece of work".
Established gambling operators fear that in the biggest review of betting rules since 2005, MPs will not be able to distinguish between them and unlicensed sites that offer betting to UK gamblers.
The government began the gambling law review late last year, which is expected to be the biggest shock to the online gambling sector.
Some of the UK's best known brands like William Hill, Sky Bet and Paddy Power have invested millions of pounds in safer gambling.
And licensed bosses fear that crackdown will drive gambling underground, leaving problem gamblers less than more exposed.
Mr Dugher said, "There is no regulated sector consumer protection on illegal sites, such as rigorous ID and age checks, safer gambling messaging and the ability to set deposit limits."
Critics argue that Blairite's online gambling liberalization has gone too far and licensed operators are exaggerating the number of their unlicensed counterparts in the hopes that regulation will focus on them.
Mr. Dugher added, “I know this evidence is impractical for those who try to deny and downplay the black market threat, but there is a real risk of complacency.
“Britain is risking sleepwalking in changes in which the main beneficiary is the unlicensed black market. We all have an interest in getting future changes right. So we need to take this latest knowledge into account and look at what is happening elsewhere in the world. "
Culture Minister Nigel Huddleston has promised to include the black market in the review of the gambling law.
Ministers have already promised to raise the minimum age for playing the National Lottery from 16 to 18 years. Downing Street also wants to ban the British from buying lottery tickets in-store with credit cards, which is currently being rejected by the Department of Digital Culture, Media and Sports, according to industry sources.
In the meantime, the review will shed some light on the relationship between betting and sports.
Mr Huddleston responded to reports over the weekend that sponsorship of bets for all sports could be banned by saying "No decisions have been made yet and policy development will be demonstrable".