At peak times, the online poker room has 65,000 punters playing

Analysts believe Partygaming - whose site is the world's biggest - could be valued at more than �5bn, making it London's largest listing in four years.

Should it reach that valuation, Partygaming would be worth more than household names such as Boots, Sainsbury's and British Airways.

The firm is the latest to try and tap into booming demand for online gaming.

Card school

Partypoker has regularly been attracting 70,000 simultaneous players at peak times, the company said.

Gibraltar-based Partygaming was launched in 1997 and made a pre-tax profit of �371m last year.

During the first three months of 2005 earnings growth has continued and sales almost doubled from the same period a year earlier.

PartyGaming is a highly profitable and cash generative businesses Richard Segal, PartyGaming The profits and perils of online poker

"Partygaming is a highly profitable and cash-generative business," said its chief executive Richard Segal.

"Our focus will be to deliver attractive returns for our shareholders through a combination of the growth of the business and through the payment of dividends."

The company said the money raised in the flotation would be used to fund acquisitions, develop new products and expand its business into new countries.

Founders Anurag Dikshit, husband and wife Ruth Parasol and Russ DeLeon, and Vikrant Bhargava, are set to reduce their holding in the Partygaming to close to 77%.

Further listing

Alongside Partygaming, a second gaming company also announced its intention to list on Thursday.

Empire Online, an online poker services firm, plans to list on the smaller AIM market on 15 June.

The company provides marketing services and helps push traffic to online gambling sites.

It has not said how much it is hoping to raise, but some estimates say it could be valued at more than �550m.

Empire Online is mainly owned by Noam Lanir, an Israeli businessman.

The company's non-executive chairman is Leonard Steinberg, founder and non-executive chairman of UK casino operator Stanley Leisure.