Vote Leave hopes to use the contest to maximise the spread of its message by taking contact details and voting intentions from hundreds of thousands of armchair punters.
It was dismissed as a con by rivals pushing for a Remain vote after experts calculated the chances of winning the jackpot were somewhere in the hundreds of trillions to one.
Vote Leave declined to say how much it had paid Lloyd’s of London to insure against someone defying the near-impossible odds.
The campaign has massively outstripped its opponents in terms of raking in private funds.
Two donors have given cash specifically to fund the contest — which also includes a guaranteed €66,000 prize fund to be shared between those who go longest without one of their projections proving wrong.
Entrants have until 8pm on June 9 to sign up to make predictions via a website.
They must supply their telephone number, email address and home address to qualify for the prize.
They must also rate themselves on a scale of one to five as to whether they are more likely to back maintaining membership or divorce from Brussels.
The competition, open to all registered voters over 18, is based on a similar competition run by US billionaire Warren Buffett involving the ‘March Madness’ basketball in the US.
To qualify for the €66m, someone would have to correctly predict in advance the outcome of each group game, the line-up at the knockout stage and each subsequent game up to the final in Paris on July 10.
Will Straw, executive director of Britain Stronger In Europe, said it was a con offering “the same length of odds as Vote Leave coming up with a coherent vision for what life would look like outside the EU.
“Given the £50m [€66m] figure is a work of fiction, this is a political Ponzi scheme with Dominic Cummings acting as Vote Leave’s Bernie Madoff”, he said in a reference to the jailed US financier.
“Once again Vote Leave put wild guesswork at the heart of their campaign and it is the British people who they are asking to take the risk.”