William Hill fell 8.6% to 284.70 pence each, while Ladbrokes Coral dropped 7.9% to 117.1 pence. Both are constituents of the FTSE 250 mid-cap index.
Britain's Times newspaper Thursday reported that a group of lawmakers will demand tighter rules on betting machines that allow players to lose a significant amount of money in a short amount of time. The members of parliament will publish a report following a six-month investigation into the machines later today.
The document, which is supported by lawmakers from all main parties, makes a case of cutting the stake on these machines to as low as £2 from the current maximum level of £100, and slowing down the speed in which the gamblers can place their bets from once every 20 seconds.
Shares in Paddy Power Bet (PDYPY) , whose business is done mainly online, edged down 0.7% to 8,175 pence each by 09:45 GMT.