Shares in gambling companies have plunged in value by hundreds of millions of pounds after the industry regulator said it would consider slashing the maximum allowable stake on online casino games to £2.

The Gambling Commission’s chief executive, Neil McArthur, told a cross-parliamentary group of MPs (APPG) investigating the harm caused by betting that it would consider their proposal to cut stakes over the next six months.

When markets learned of his comments on Thursday morning, investors in bookmakers and online casino firms staged a sell-off.

William Hill fell nearly 8%, while Ladbrokes Coral’s owner, GVC, and the gambling software firm Playtech were both down 7%. Rivals such as Flutter and 888 fell by smaller amounts.

Between them, UK gambling firms’ stock market value fell by more than £500m.

A spokesperson for the Gambling Commission said: “We said last October that we would be looking at online stake limits as part of our ongoing work to reduce the risks of gambling-related harm.

“This work is in addition to us focusing on VIP practices, advertising technology and game design. We will publish our assessment and next steps for online stakes and further protections later this year.”

McArthur promised to review online stakes during an evidence session with the increasingly influential all-party parliamentary group on gambling harm.

The group was instrumental in garnering cross-party support for similar restrictions on fixed-odds betting terminals that ended with maximum stakes on the controversial machines being cut from £100 to £2.

The curbs in effect amounted to a ban on the machines, which were criticised for enticing gambling addicts, because they are uneconomic for high street bookmakers at that level of stake.

Thursday’s share price fall was the second time the APPG’s intervention has led to hundreds of millions being shaved off the UK gambling industry’s stock market value.

It shed nearly £1.2bn in November after the Guardian revealed MPs had recommended stringent curbs on online casino games worth more than £2bn a year to the industry.

The government has said it will review the 2005 Gambling Act, introduced under Tony Blair’s Labour administration, amid growing concern about gambling addiction and transgressions by the industry.

There has been greater focus on issues such as the effect on children and vulnerable people of the volume of gambling advertising, the industry’s ties with football and the use of controversial VIP schemes to offer perks to losing players.