Centenarian Frances Hilton is promoting the campaign

Energywatch estimates that 750,000 people who are most in need of support have never heard of the Priority Service Register (PSR).

Energywatch is launching a campaign on Monday to promote PSR, which entitles over-60s and people with disabilities to a range of free services.

These include meter readings and secure passwords to thwart bogus callers.

The campaign is being backed by Age Concern, the Disabled Living Foundation and the government's Pension Service.

Centenarian Frances Hilton, who signed up to the service a year ago, is helping promote the campaign.

"I'm on the special password protection scheme which makes me feel a lot safer because it lets me know the person at my door is genuine," Mrs Hilton said.

"I've also been given special fittings on my appliances to make things easier to use."

PRIORITY SERVICES REGISTER The register is a list of elderly, disabled (including customers who are blind or partially sighted, deaf or hard of hearing) and chronically ill customers, who may need additional help Each domestic electricity and gas supplier is obliged to maintain a register of these people and offer special help to customers Benefits include "easy read" bills, a scheme to deter bogus callers, and priority reconnection after a power cut Source: Ofgem Changing energy supplier Disconnections - your rights

One million people have so far signed up to the register - just over a quarter who are eligible.

Of the three million remaining customers, Energywatch estimates 750,000 are most at need of joining the service.

"The energy industry needs to work together to take action which will give the PSR a much higher profile to ensure that consumers don't continue to miss out," said Allan Asher, chair of Energywatch.

Although the government has set targets to cut fuel poverty, energy firms are not obliged to offer widespread support to vulnerable users, although many do.