Shares in online casino group PartyGaming surged on Monday after it completed the biggest London flotation in five years, taking its market value to about £5 billion ($9.1 billion).

The over-subscribed offer values PartyGaming higher than many British household names such as British Airways and chemicals group ICI.

PartyGaming priced its initial public offering at 116 pence per share, giving a market value of £4.6 billion and guaranteeing a place in the FTSE-100 index of blue-chip UK companies.

The stock traded as high as 132-1/2p before closing 11 percent higher at 129p.

Today's flotation has made more than $1 billion for a former US pornography entrepreneur, her lawyer husband and two Indian technology graduates.

The four brains behind the world's biggest online poker operator, which was founded in 1997, will cash in shares worth about $1.4 billion in total in the initial public offering, and keep stakes worth over $6 billion at current valuations.

But the windfall does not come without risk. PartyGaming's listing prospectus said its directors risk jail if US moves to clamp down on online gaming are successful, resulting in potential criminal or civil action.

Founder Ruth Parasol made her fortune originally from running premium rate adult chat lines and adult entertainment websites, before switching to online gaming in 1997.

Parasol and her husband Russell de Leon, both lawyers, are each set to cash in around $370 million and still own a combined stake worth about $2.7 billion. Parasol and de Leon now act as legal consultants to the Gibraltar based company.

Anurag Dikshit, group operations director, is the biggest shareholder. He will cash in shares worth about $720 million and still own a 30% stake worth over $2.6 billion.

Marketing director Vikrant Bhargava's remaining 9% stake and cash from the IPO should be worth almost $1 billion.

The four major shareholders, all in their 30s, are famously publicity shy, despite being thrust into the limelight by the flotation.

One hugely successful move was the launch of a global tournament, where players compete online before finalists meet on a cruise and the winner takes home as much as from big casino competitions. This year the winner scooped $1.5 million.

PartyGaming, which does not take a risk in the game, now reaps a profit of almost $1,000 every minute as it pulls in revenue of $100,000 every hour of the day. Up to 70,000 people play simultaneously on its sites.

Investment bank Dresdner Kleinwort Wasserstein is also set to earn the bulk of up to $61 million expected to be paid to the IPO advisers.

The Gibraltar-based firm had profits of £371m in 2004.