Up to 125,000 households out of the 3.5 million with the least efficient types of boiler could benefit from the £50m scheme.

Only people who own their homes or landlords who rent out property are eligible but to qualify households need to have a working G-rated boiler. It is likely to be G-rated if it is more than 15 years old and gas fired or 25 years old and oil fired.

If in doubt the presence of a permanent “pilot light” is also a good indicator of a G rating.

People aged below 60 can only apply if the boiler is the main boiler used to heat the home and is in working order. For those aged above 60, the device does not have to work.

Householders will have to supply a quote from a registered installer to the Energy Saving Trust (EST) with details of the old boiler and the proposed installer and it will then issue a voucher worth £400 which can be redeemed once the work has been done.

Energy companies are expected to use the scheme to match the government's £400 discount.

The government says the scheme will help to safeguard jobs across the industry, which employs in the region of 130,000 people and save about 140,000 tonnes of greenhouse gas emissions every year.