City firm Rowe & Maw stands to earn fees worth millions of pounds from the National Lottery if Richard Branson succeeds in his bid to run it.
The firm has won work advising leading US lottery group Automated Wagering International (AWI), which is supplying the technology for the gaming terminals and central computer system in The People's Lottery bid.
Rowe & Maw was chosen after being invited to enter a beauty parade against three other City firms. AWI general counsel Dena Rosenzweig will not name the competition.
Rowe & Maw's head of IP and IT Stephen Gare led the pitch alongside corporate partner Andrew Stewart.
Gare says the work is a big win for the firm.
He has already started work for the client, putting together the details of the bid, which had to be submitted last week.
Gare says: "For the last few weeks, I have devoted most of my time to this. But from now on we will have up to 12 lawyers on it and it will take up 25 per cent of my time. It's a big IT job.
"The ability to resource a project of this size in just a few weeks was a key factor which influenced AWI's choice of firm."
If The People's Lottery wins the bid the firm will have an eight-year relationship with the client. Gare says it is too early to say how many people will need to be working on it at that stage.
Rosenzweig says: "Rowe & Maw impressed me because it focused on what we wanted to achieve commercially and moved very quickly."
London media firm Harbottle & Lewis is acting for The People's Lottery in the bid, with company partner Colin Howes and IP partner Mark Owen leading a 10-strong team. The firm won the work after advising Branson on his 1993 bid to run the lottery.
Nabarro Nathanson and CMS Cameron McKenna are advising the Post Office and Camelot respectively in their joint opposing bid. A decision is expected in June.