Labour said the energy regulator had warned of a ‘systemic risk’ to supply 18 maande gelede.
Ministers have denied being complacent over 18-month-old warnings about risks to the UK’s energy supply after 1.5 million people were left without a provider.
A host of energy companies have gone to the wall in recent weeks after the sector was hit by rocketing global wholesale gas prices.
Met 800,000 consumers losing their suppliers on Wednesday alone, two energy companies have since looked to make it more difficult for new customers to sign up for their services as they attempt to survive the current turbulence.
Bulb scrapped its popular refer-a-friend scheme as it tries to raise new cash, while rival Ovo Energy changed its website by removing an invite to “get an energy quote in under two minutes”.
Opposition leader Sir Keir Starmer tweeted that the details show the Government was “warned about a looming crisis and didn’t prepare”.
Speaking in the Commons, Mr Miliband accused ministers of being “complacent” about the shock that rising gas prices could wreak upon the market, as well as families and the cost of living.
But Mr Kwarteng said the Conservative administration had “not been complacent” as suppliers collapsed.
The Cabinet minister said Ofgem’s concerns had been “interrogated” during the coronavirus pandemic, with the supplier of last resort programme, where consumers are automatically transferred to a new provider if their supplier exits the market, was “found to work”.
It comes as a minister in Mr Kwarteng’s department admitted the energy price cap is under “pressure” due to crippling energy prices.
Maar Paul Scully said it would be up to the energy watchdog to decide if the cap, which he said is saving dual-fuel energy customers around £100 per year, should rise.
The cap is currently due to next be reviewed in April.
The business minister told Sky News the Government was “planning for the worst-case scenario” which he said was that gas prices would remain high beyond a “short spike”.
Both Mr Scully and Mr Kwarteng repeated the assertion on Thursday that the energy price cap would remain and that they did not want to bail out firms on the brink of failing.
Euro, met 580,000 domestic gas and electricity customers, became the biggest of a series of recent failures in the energy market on Wednesday, with the number of collapsed companies accounting for more than 5% of the market.
Green – which also closed on Wednesday – PFP, MoneyPlus, Utility Point and People’s Energy have all exited the supply market in just over two weeks.
HUB Energy, which had 6,000 kliënte, stopped trading last month.
Ofgem has said it will ensure a new supplier is appointed to take over Avro’s clients and Green’s 255,000 huishoudings.
Conservative MP Mr Scully said the regulator would ensure “continuity of supply” for any other consumers whose providers go to the wall so they can keep the lights on.
But he said the Government could give no guarantees that those being switched to new providers would not have to fork out for a higher tariff than they were paying before, admitting some people are facing a “tough” winter ahead.
His comments come only two days after his boss Mr Kwarteng said those being transferred would “be expected to pay the same amount”.
In the Commons, the Business Secretary acknowledged people living in the north of the country will be hit hardest by energy hikes due to the colder weather.