Shares in Rank Group have fallen 7% after the firm said that changes to casino gaming duty announced in the Budget would cost it £8m a year.
The chancellor scrapped the 2.5% starting rate for the smallest casinos, raised the 12.5% rate to 15% and set up a new band paying 50%.
The 50% rate will be paid on profits of more than £10m, which should include the new super-casino in Manchester.
Rank shares closed down more than 7% at 208.25p on Thursday.
Rank said the changes were "imposed without prior indication or consultation with the industry".
Casino operators had been calling for an easing of gaming duty to help offset the effect of smoking bans.
There was also bad news for any off-shore internet gaming sites that were considering relocating to the UK.
Internet gaming will be subject to a 15% duty, the same as High Street bookmakers and bingo halls.
The government had previously expressed the wish to become a centre for regulated remote gambling.
But online firms, which are mostly based offshore in tax havens, had been lobbying for a rate of 5% or less if they were going to move to the UK.