Shares in UK betting firms have fallen after a cross-party group of MPs called for measures to overhaul online gambling and protect vulnerable people.
The MPs have called for a limit on the maximum stakes on online slot machines, similar to the £2 limit brought in for fixed-odds betting terminals (FOBTs).
They are also seeking a ban on the use of credit cards to gamble online.
The group also says it wants a far more responsible approach to advertising and restrictions on VIP accounts.
These accounts can often involve offers of bigger rewards to frequent gamblers.
The Gambling Related Harm All-Party Parliamentary Group's recommendations include calling for treatment of gambling addiction to be offered under the National Health Service (NHS).
News of the report sent shares in GVC - whose brands include Coral and Ladbrokes - down 7.7%, while William Hill fell 7.5%.
"Unlike the land-based gambling sector, there are no stakes, prize or deposit limits in the online gambling world. It is possible to bet thousands of pounds in a matter of minutes from a mobile phone with no supervision," the MPs' report said.
It added that it was "now a matter of urgency that stake and deposit limits are introduced in online gambling to reduce the harm that the industry is causing".
"[The government] agreed that the way to limit the harm from fixed-odds betting terminals was to limit the stake to £2. It is abundantly clear that stake and deposit limits are needed in the online world to limit harm.
"We do not see the justification for having slot machine style games online with staking levels above £2. If they are not acceptable in land-based venues they should not be allowed online."
Conservative MP Iain Duncan Smith urged the Gambling Commission to look into the issue in greater depth.
"For too long, online gambling operators have exploited vulnerable gamblers to little or no retribution from the regulator," he said.
UK betting firms are still coming to terms with the government's crackdown on FOBTs, which cut maximum stakes from £100 to £2.
As a result, William Hill has said it plans to close 700 UK betting shops while GVC expects to shut about 900 outlets.