Good causes have taken a £1.6bn hit in the past year as National Lottery ticket sales nosedived.
That represents a 15% fall in the money going to help various projects, sports clubs and community efforts up and down the country.
But, profits at operator Camelot stand at £71m, up 122% from £39 just six years ago, the National Audit Office reports.
Camelot told the NAO that fewer people were taking part in the draws held each week and were instead seeking instant wins on scratch cards.
However, it also told auditors that scratch cards, where sales fell by 2% last year, gave a smaller amount to good causes than the draw, where sales fell by 13%.
Lottery sales fell by 9% to £6.93bn in the 2016-17 tax year, and Camelot predicts a further fall in sales and income for good causes in 2017-18.
Changes to the Lotto draw in 2015 saw the number of balls increase from 49 to 59 and the chance of winning the jackpot decrease from one in 14 million to one in 45 million.
The cost of playing EuroMillions also increased by 50p to £2.50 a line, and the number of “Lucky Star” numbers rose from 11 to 12 under changes introduced in September last year.
The NAO report said Camelot’s “overarching objective” was to maximise returns for good causes – but that income was predicted to fall again next year.
About 34p for each £1 spent on draw-based games bought online goes to good causes, compared with about 5p to 10p for scratch cards.
A Camelot spokesman said: “The NAO report restates what we publicly acknowledged back in June in relation to National Lottery sales and returns to good causes.
“Since then, we have carried out a wide-ranging strategic review of the business and announced strong plans to get the National Lottery back into growth next year and raising as much money as possible for good causes.”
New chief executive Nigel Railton has pledged sweeping changes to its games to get the business back on track.
The plans include changes to Lotto “to give players a better winning experience” and a new “annuity-style game” where players win regular sums of money for the rest of their lives rather than a bumper jackpot.