Poker players are increasingly being lured into illegal high-stakes games in pubs and clubs, the government's gambling watchdog fears.
Criminals could be holding all the aces, the watchdog fears
The Gambling Commission is launching a crackdown amid fears criminals are attempting to cash-in on the internet-driven boom in poker.
Landlords have been warned they could lose their licence if they allow poker to be played on their premises.
It comes amid US moves to rein in the booming online gambling market.
The US House of Representatives has backed a bill limiting internet gaming by making it illegal for US-based banks and credit card firms to make payments to online gambling sites.
Online gambling has taken off in the past two years, with the global market thought to be worth £827m.
In the UK, the Gambling Commission fears criminals are cashing-in on the boom.
It says a big increase in unregulated poker games in pubs and clubs could lead to a rise in violence - and unwary punters being cheated out of large amounts of cash.
It has written to police forces, local authorities and the licensing trade associations throughout the country as part of a clampdown.
"Poker has become increasingly popular in the last 12 months and a number of people are seeking to cash in on the demand to play," said Phil Brear, the Gambling Commission's director of operations.
"The law is complex but we have two simple messages: First under the existing law many of these games are illegal or are likely to lead to illegal games being played.
"Secondly, illegal poker games can lead to people being cheated into losing very substantial sums of money, and we are concerned that people will be exploited if games are not properly supervised."
Small stake gambling - such as on cribbage or dominoes - is allowed in pubs and clubs under the 1968 Gaming Act, the commission says in its guidelines.
But any game which involves high stakes or betting against the bank or house, such as pontoon, blackjack, roulette, poker or brag, is outlawed, it warns.
The rules are complicated, however.
Some traditional games, such as bridge, solo, whist and rummy are allowed in pubs provided the winner walks away with no more than the price of a round of drinks.
Bingo is also allowed - provided it is an "incidental activity" and the stakes are not too high.
Poker leagues - where players compete for points rather than cash - are a legal grey area, the commission admits, as the organisers often offer prizes in the later stages.
But it is illegal to play any game in a pub where the host takes a percentage of the "pot", the commission warns.
And landlords who use poker to lure punters into their premises also face prosecution or losing their licence.
Poker is a "great game", said Mr Brear, but it should only be played in licensed casinos or card rooms, with properly-trained dealers and supervisors, CCTV and other safeguards.
"Our overarching objectives are to keep crime out of gambling, protect players and make sure that the games are run fairly and openly.
"The Gambling Commission has significant powers at its disposal to crack down on those breaking the law and we won't hesitate to use those powers," he added.
Charity poker tournaments and other fund raising events may be allowed by the licensing authorities, the Gambling Commission guidelines say, provided the landlord does not make substantial amounts of extra cash from the sale of food and drink.
The 2005 Gambling Act - due to come into force in September 2007 - will include a new definition of "non-commercial gaming", including limits on stakes and prizes.