Olympic gold medallists helped launch the game

Cash from Dream Number, which will be drawn twice a week, will be used to build new venues and the Olympic Village in Stratford, east London.

For a �1 ticket, prizes between �2 and �500,000 will be available when the Camelot-run game starts on 15 July.

Hurdler Sally Gunnell and rower James Cracknell were among those at the launch in central London.

Dream Number is the first lottery draw to raise money towards the cost of hosting the Olympics. Scratchcard sales are already ahead of target.

Camelot chief executive Dianne Thompson said sales of designated lottery games in support of the London Olympics had got off to a tremendous start.

She added: "We hope that Dream Number will prove equally inspiring to our players."

Themed games

Hosting the event is expected to cost �2.375bn, and the lottery games are thought to be vitally important to the fund-raising effort.

Overall, the lottery will give �1.5bn towards the 2012 Olympics, with �750m from themed games, making it the biggest source of public funding.

Money will also come from the International Olympic Committee, TV and marketing deals, sponsorship and official suppliers, ticket revenues and licensing.