Graphic designer Cliff Bryant placed two £5 accumulators on snow falling across 24 British towns and cities on Christmas Day.
However, although the Southampton 52-year-old's weather forecasting proved accurate, his understanding of Ladbrokes' betting rules proved less so after it emerged that his wager should not have been accepted.
Ladbrokes spokesman David Williams said: "We apologise to Mr Bryant and regret that his bet was taken in error.
"We do not take accumulator bets on events such as snowfall because if snow falls in one city, it is likely to fall in another. We will of course settle Mr Bryant's legitimate bets and have offered to put him in touch with the Independent Betting Arbitration Service if he wishes to take the matter further.
"With over 14,000 retail staff across the UK, occasional mistakes do happen, but we will remind our staff of betting rules on novelty events to avoid thishappening again."
Bryant, whose two accumulator bets are reported to have been worth £4.9m and £2.23m, told the Southern Daily Echo: "This is a genuine mistake and if I make a mistake in my work like that it costs me dearly and I think the offer should be a lot more generous than they have made.
"They are one of the leading bookmakers in the country and I think they ought to do their homework a bit better in future."
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