Six people died and more than 300 have been left injured in the last year as a result of gas-related incidents.
The figures were revealed by the Gas Safe Register, which said it found dangerous appliances in one in six of the 142,507 homes it had inspected over the past five years.
People are being warned of the dangers of unsafe appliances as part of Gas Safety Week, which runs until Sunday, September 20.
Across the UK, the West Midlands had the highest number of homes with unsafe gas appliances, where one in four inspected were found to be unsafe, compared with the East Midlands, which had the fewest number of homes with unsafe gas appliances, equivalent to one in nine.
Jonathan Samuel, chief executive of Gas Safe Register, said: "With the average cost of safety checking your gas boiler at just £60, it's a small price to pay to keep you and your family safe and prevent damage to your home.
"While we're seeing more people getting gas safety checks compared to last year, millions are still taking risks so we're reminding people to make sure all their gas appliances are safety checked every year."