The biggest shareholder in PartyGaming, the UK-listed online gambling company, has agreed to pay US authorities $300m 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 is due to appear in the Southern District Court of New York on Tuesday 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.