New guidelines say installations should also not take place where people have severe health issues - including terminal illness - and in the homes of people aged 85 and over who live alone.

Customers with a condition that would be made worse by the cold - such as emphysema - will also be protected, as will those who need electricity for medical kit.

However, critics say vulnerable people are still unprotected by the changes and that the Ofgem code of conduct is only voluntary - with the Lib Dems calling it an "insult" and a "half measure".

The changes come after an investigation found debt collectors had forced their way into homes to install prepayment meters.

In February, energy watchdog Ofgem asked suppliers to temporarily suspend the practice.

All of Britain's domestic energy suppliers have signed up to the Ofgem code of conduct and installers will have to wear cameras to check they are complying.

Read more:
How do prepayment meters work and what are the rules around them?

Prepayment meters are installed when a customer is in debt and are pay-as-you-go, meaning power is cut off if payment isn't made.

Three million people ran out of credit on their prepaid meter last year, according to Citizens Advice, and more than 94,000 were forcibly installed.

However, prepayment charges can sometimes be more expensive than regular bills.

The new Ofgem code also states that £30 credit should be given when a meter is forcibly installed to prevent the risk of people being immediately cut off.

Once a customer has repaid any debt, their case must now be reassessed to see if prepayment is still the best option.

Forced installations should also not happen when there's no one in the household with the physical or mental capacity to top up the meter.

Ofgem said it will closely monitor firms to make sure they stick to the new code and that companies must set out a plan on how they will implement it.

Firms also need to conduct an audit to identify customers who had prepayment meters wrongfully installed.

These people should be offered compensation and a return to regular billing.

'An insult - with vulnerable people still at risk'

Ofgem chief Jonathan Brearley said the changes make clear that prepayment "must be as a last resort, and customers in vulnerable situations will be given the extra care and consideration they deserve".

The regulator said it would now consult on whether to incorporate the updates into firms' licence agreements - which would importantly make it legally enforceable.

Citizens' Advice said it was a "much-needed improvement" and that "it's now up to suppliers to follow the rules and for Ofgem to crack down quickly on any sign of bad practice".

But its boss, Dame Clare Moriarty, warned that Ofgem "must also act swiftly to make this voluntary code mandatory".

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However, the Liberal Democrats said the changes were woefully insufficient.

"This half-measure is an insult to all the people who had their homes broken into by big gas companies," said Wera Hobhouse MP, its energy spokesperson.

"These new plans will leave too many vulnerable people, retired pensioners and hardworking families unprotected.

"Predatory prepayment schemes cannot go on a moment longer. No one should need to pay a poverty premium because of this government's incompetence."

Energy Secretary Grant Shapps indicated the changes could be toughened up, replying "Yes, we're on it" when asked by Sky News if more could be done to control pre-payment meters.

The charity Scope also claimed they could still be fitted in some disabled households under the new code.

"We want to see the forced installation of meters and remote switching banned outright for disabled people," a spokesperson said.

Simon Francis, from the End Fuel Poverty Coalition, said the change "simply does not go far enough and the fact it is voluntary undermines its objective".

He said "really vulnerable groups" had been "omitted from its full protection".

"We have serious concerns about how it will be implemented, such as how people will prove their medical conditions without being humiliated by an energy firm health inspection," added Mr Francis.