Online casino operators are failing to prevent money laundering and protect problem gamblers, the Gambling Commission has warned.
It expressed "significant concerns" in a letter to all 195 UK operators and warned them to review procedures.
The commission said many were unable to spot signs of financial crime and compulsive gambling.
An investigation has begun into 17 companies, with five under licence review.
If a company loses its licence, it would no longer be able to operate in the UK.
A third of all gambling in the UK now takes place online.
But despite the high numbers, BBC business correspondent Jonty Bloom said the commission found evidence that customers whose gambling was becoming compulsive were not being followed up.
He also said that some staff had little idea of how to spot money laundering – or, in some cases, even what it was.
In its letter, the commission said all companies should ensure staff are trained in the law and that operators should carry out risk assessments – such as monitoring and risk profiling of customers.
Sarah Harrison, chief executive of the Gambling Commission, said the organisation was taking action to make the market "fairer and safer" and it would be "relentless in turning that vision into reality."
She added: "It is vital that the gambling industry takes its duty to protect consumers and keep crime out of gambling."