Labour yesterday attacked the Big Six for disconnecting tens of thousands of customers “by the back door”.

Representatives of the suppliers however claimed they did not disconnect customers over the winter months while appearing before the Energy and Climate Change Select Committee back in November.

But Labour said the release of written responses of the Big Six to questions raised during the session shows that’s not the case for their pre-pay customers, who are forced to “self-disconnect” when they can’t afford to top up their meters.

It added: “Tens of thousands of vulnerable customers on pre-payment meters could be left without gas or electricity for as long as three months before they receive assistance from their energy company.”

According to figures from npower more than 23,000 of its pre-pay customers went for longer than 30 days without topping up their meter in 2012. Yet the company waits 90 days before getting in touch.

EDF didn’t say how many of its customers went 30 days without putting money in the meter last year but said it also waits 90 days before making contact.

Scottish Power and E.ON both leave 35 days before putting a call in and British Gas and SSE are quickest off the mark – phoning up after 28 days.

As members of Energy UK, the suppliers have all signed up to the group’s ‘Safety Net’ pledge, which says they will “never knowingly disconnect a vulnerable customer at any time of year, where… that customer is unable to safeguard their personal welfare or the personal welfare of other members of the household”.

Responding to the new figures, the trade body said: “No one should be afraid to turn on the heating or worry they cannot pay for the energy to cook a meal.”