Partygaming, owner of online poker firm Partypoker, is to raise up to £1.1bn ($1.9bn) in its initial share offer.
Online gaming is growing in popularity
The firm will seek a London stock market listing later this month in what will be the City's biggest flotation for almost four years.
Shares will be priced at between 111 pence and 127p, valuing the firm at between £4.4bn and £5.1bn.
The firm, which runs the Partypoker website, is the latest to try to tap into booming demand for online gaming.
Trading is expected to begin in the stock on the London Stock Exchange on Monday, 27 June.
Spread betting firm IG Index said that the bid price of the shares in the unofficial grey market was 115p.
The firm had been expected to be valued at as much as £5.8bn, but Partygaming denied it had cut its price after a lacklustre response from investors.
Experts have suggested investors have been wary about the firm amid concerns about the legality of online gaming in the US, where 80% of its players are based.
But fellow internet gaming group Empire Online fared well at its London market debut, which coincided with Partygaming's announcement.
Shares in Empire Online - which markets mainly gambling websites - were placed at 175p, valuing the firm at $512.4m.
However, shares in the group did end the day 1.5p lower at 173.5p.
The move is also expected to propel Partygaming into the UK's leading FTSE 100 index.
The share sale is set to be the biggest in the City since technology company Dimension Data in 2000, while the size of the initial public offer is the largest since Friends Provident in 2001.
The flotation will give its 1,000 staff a 5.6% share in the group and bring huge windfalls for its four owners.
Partypoker has regularly been attracting 70,000 simultaneous players at peak times, the company said.
Richard Segal, chief executive of Partygaming, said: "Whilst the group is already a substantial business, a listing will enhance the visibility of the group and its brands among all our stakeholders, including customers, partners and employees, at a time when Partygaming is seeking to expand internationally."
Gibraltar-based Partygaming was launched in 1997 and made a pre-tax profit of £371m last year.
It says it has a 55% share of the online poker market.
The company added that revenues jumped 93% in the first three months of the current year to $222m compared with the same period last year, while operating profits rose 81% to $128m.