Miliband said he and the chancellor were “really concerned” over what was happening in some parts of the market and had met the CMA earlier in the week to discuss heating oil and motor fuels specifically.
“They’re looking at the situation carefully. They are willing to intervene,” he said.
“We will not tolerate unfair practices, price gouging.”
The CMA has a range of powers including fines they can impose on firms.
“It’d be completely unacceptable for anyone to use this crisis, to rip people off,” Miliband said. “And we will fight people’s corner to stop that happening.”
However, whether the government took further steps to support households would depend on how long the conflict lasted, he added.
He emphasised that the chancellor had previously shown a “willingness to intervene”, a reference to moves in November’s Budget to ease energy bills, with additional money targeted at vulnerable households.
Fuel duty, which is currently frozen, is set to rise in September. That was now under review, Miliband confirmed.