Chloe Smith last night struggled to answer questions about the decision to postpone the 3p rise in petrol duty.
After she refused to say when she found out about it or how it will be funded, the Economic Secretary was accused of being "incompetent" by Jeremy Paxman, the BBC's Newsnight interviewer.
Mr Paxman also asked her whether she ever woke up and thought: "My God, what am I going to be told today?"
She replied: "I wake up in the morning and know, actually, that some of my constituents will really value not having to pay that little bit more on fuel prices come August because the cost of living is pretty tight at the moment and everybody does know that."
The Prime Minister's spokesman today confirmed that Mr Osborne alone took the decision to scrap the rise due in August. Ministers were not told about the move before he announced in it Parliament on Tuesday.
Nadine Dorries, a backbench Conservative MP, accused the Chancellor of being a "coward" for sending his junior minister to deal with questions.
She launched a scathing attack on Mr Osborne on Twitter, the micro-blogging website, branding him a "submarine Chancellor" who goes to ground at the first sign of trouble.
"I was at a dinner last night so didn't see Newsnight, however, if Osborne sent Chloe on re scrapping 3p he is a coward as well as arrogant," she wrote.
"Newsnight last night would have been a tough gig for a Minister with years of experience - Chloe is a good egg and didn't deserve that. The submarine Chancellor sacrifices another Minister whilst he slips under the surface...again."
Ben Bradshaw, the former Labour minister, also waded into the row to say the interview was "proof if you needed it that George Osborne is not a gentleman forcing poor Chloe Smith to crash on Newsnight".
Louise Mensch, another Conservative MP, rallied round her colleague, saying "anybody can have a tough time with Paxman" as Newsnight is "hard stuff".
The Prime Minister faced questions about why the Chancellor let Ms Smith do the interview during his weekly questions in the House of Commons.
David Cameron said there was no question the Chancellor was "hiding away", as he had gone to the House of Commons to tell MPs the rise would be scrapped.
"He was actually here making the announcement," he said.
Ahead of his appearance, Maria Miller, the minister for disabled people, also struggled to answer questions about the rise on the BBC's Daily Politics.
She defended the decision to put up Ms Smith for the interview, saying it is "absolutely right" that ministers take responsibility for Government policy.
