Martin Lewis said the "zombie Government" must not wait until the conclusion of the Tory leadership contest to decide on help for households who will face the energy price cap rising to £3,500 or more.
The Money Saving Expert dismissed the extra help promised by Mr Sunak and Ms Truss during their leadership bids as "trivial" in the face of bills which are set to be £2,300 a year higher than they were last October.
He said: "I've never seen anything like this. It's going to throw many households into a terribly difficult financial situation that will leave them making some awful choices."
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He told BBC Radio 4's Today programme the data that informs the price cap suggests it will increase 77% on top of the 52% rise in April, taking the typical bill to £3,500 a year.
Mr Lewis warned: "Others say it will be higher. We are expecting it to rise again in January."
Mr Lewis said the choice facing the Government is "you either have to cut prices for people or you have to put more money in their pockets, especially at the poorest level".
But he added: "The problem is we have this zombie Government at the moment that can't make any big decisions."
Major policy decisions have been postponed until the new prime minister takes office, with the Tory leadership contest scheduled to conclude on September 5. Mr Lewis said: "You have to look at the timing issues here. The formal announcement of the price cap will be towards the end of August."
That would trigger companies to inform their customers about an increase in their direct debits. Mr Lewis said: "People will be panicking, it will be desperate - they are already panicking right now.
"By September 5 when we have a new prime minister, we will already be absolutely in the mire of this."
In a message to Mr Sunak, Ms Truss and Mr Johnson, he said: "Please, go and sit in a room together, make a collective decision now of what help you can give and make an announcement to forestall the mental health damage that is coming across the country.
"There needs to be action now, you are all in the same party, you should be able to work out some unifying policy, something for heaven's sake. Sit in a room, decide what you are going to do together, take a little bit of collective action and give the panicking people across the country a little bit of respite from this."
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