The biggest shareholder in PartyGaming, the UK-listed online gambling company, has agreed to pay US authorities $300m (€219m) and plead guilty to a charge relating to illegal web betting in the most prominent prosecution so far in the US clampdown on internet gambling.

Anurag Dikshit, co-founder of the company, retains a 27 per cent stake and will appear in the Southern District Court of New York today to admit to an offence under the Wire Act and to agree to co-operate with the US Department of Justice, people close to the situation say.