Tens of thousands of vulnerable gas and electricity customers could be going without power for up to three months, despite promises from the big six companies that no one will knowingly be cut off. The Labour party argues that an unacceptable "disconnection by the back door" has taken hold because many on prepayment meters are not receiving early help from suppliers.

Caroline Flint, shadow secretary of state for energy and climate change, said the situation underlined the wider cost-of-living crisis and reinforced the need for wholesale reform of the energy sector.

Npower is one of the companies that in 2013 had more than 20,000 customers on its books who had not topped up their meters for at least 30 days. The company has a rule that it will contact any such customer after 90 days, although on Saturday it said it would change this policy in the coming months.

EDF had almost 200,000 electricity and more than 140,000 gas accounts last year where no payment was made for at least 30 days, although the vast bulk of these are believed to involve vacant properties. It also waits three months before contacting customers.

The plight of those on prepayment meters is already under scrutiny, with recent figures from the Department of Energy and Climate Change showing this category of energy user is paying on average about £100 a year more for gas and electricity combined, compared with those on direct debit. There is further concern because some of those on prepayment schemes have been pushed on to them by energy suppliers as a result of past debts, argue fuel poverty campaigners.

"David Cameron has failed to stand up to the energy companies and these figures show that tens of thousands of vulnerable customers hit by his cost-of-living crisis could be going without heating in the coldest months of the year," said Flint. "It's more obvious than ever that the energy market doesn't work for ordinary families. We need a Labour government to reform the market, replace Ofgem with a new regulator with real teeth, and implement Labour's price freeze, which would save money for 27 million households."

The suppliers' trade body, Energy UK, says its members do all they can to help with fuel poverty and have signed a Safety Net initiative "pledging to never knowingly disconnect a vulnerable customer at any time of the year".

Appearing before the energy and climate change select committee before Christmas, Npower, EDF and others insisted they did not disconnect customers during the winter months. British Gas said it had not disconnected a customer for four years.

However, the figures provided afterwards by the same companies in writing to the chairman of the committee, Sir Robert Smith MP, indicate customers could in effect be disconnecting themselves, said Labour. While Npower and EDF wait 90 days before they contact the customers, others, such as Scottish Power and E.ON, wait only 35 days and British Gas 28 days.

Npower only monitors prepayment meter accounts for gas over the winter months but electricity ones all year and says it is difficult to know how many of the 20,000 customers have got an excess of credit and how many could be deemed to have self-disconnected.

A spokeswoman said: "No one should have to stop using their energy supply. If anyone is struggling with their energy bills they should contact their supplier and let them know. Every major energy supplier offers assistance for those who are most in need. From the summer we will contact customers after 30 days to check and see if they require any help."

EDF said it too has a range of measures in place to pick up those in trouble. A spokeswoman said: "We routinely contact our most vulnerable prepayment meter customers to check everything is OK. This is done through a combination of letters and telephone calls.

By regularly contacting these customers, we are able to more quickly recognise any issues with payments a customer may be having and do what we can to help them.

"With regard to some of our prepayment meter customers not using their meters for 30 days or more, as a supplier, we do not have access to real time data and only receive historical information from a meter when it's topped up. We are not privy to when a prepayment meter is at zero, or any other level. Smart metering will make this data available."

The big six are already facing calls from Ofgem and the Competition and Markets Authority for a full inquiry after a preliminary investigation highlighted a number of serious concerns. A final decision to proceed with this is expected next month.