Internet casino firms are facing a UK government clampdown over concerns some are breaking strict advertising laws.
The government says it plans to target advertisers and publishers
Culture Secretary Tessa Jowell plans to stop gambling firms from advertising free entry to online tournaments or indicating the value of prizes.
Current laws allow internet casinos to promote their brands but forbid them from offering inducements to gamble.
Ms Jowell said adverts in newspapers, on London Underground platforms and on taxis had flouted these regulations.
Unlike traditional casinos, many internet casino operators are based offshore and are not subject to UK laws.
However, the culture secretary said she planned to target the advertisers and publishers in the UK who work with them.
"It's clear that some adverts are breaking the law," Ms Jowell said.
"I am not willing to turn a blind eye to this and have agreed with the Gambling Commission that we should crack down on advertisers and publishers who knowingly break the law."
News of the proposed government action comes at a sensitive time for online casino operators, with concern over the prospects for growth in the industry denting confidence among some investors.
However, internet gambling company Partygaming said it welcomed any action that dealt with companies that broke the law.
"We are fully supportive of these measures," said John Shepherd, Partygaming's director of corporate communications.
"We are fully compliant with the law, and we are very strict about it. Our legal team and staff are well trained - we have never had one of our adverts stopped."
The Department for Culture Media & Sport (DCMS) said it planned to send letters to advertisers, publishers and gambling firms, warning them that the government would no longer tolerate advertisements for internet casinos that offered inducements.
"The letter will clarify what the legal status is. We will be saying if you break the law you will be prosecuted," a DCMS spokesman said.
Advertisers or publishers found guilty of breaking the law could face fines of up to £5,000 or two years in prison, the spokesman said.
"There have been a number of instances when adverts have crossed the line, " he said. "We are trying to protect vulnerable people from being induced to gamble."
The DCMS said the warning letters would be sent jointly with the Gambling Commission, which regulates the UK industry.