Treasury Select Committee chair claims retailers not customers benefitted from the tax cut.
Petrol stations and retailers have "pocketed" the benefit of a 5p fuel duty price cut, the chair of the Treasury Select Committee has claimed.
Conservative MP Harriet Baldwin accused retailers of not passing on the tax reduction to drivers.
The UK's competition authority has found that drivers paid an extra 6p per litre for fuel at supermarkets.
It said on Monday retailers had passed on the fuel duty cut to customers, but had charged more than they should.
"The thing that annoys me from a Treasury point of view is that the Chancellor cut fuel duty by 5p to help families with their cost of living and yet it doesn't seem to have been passed on.
"The CMA highlights that it actually 6p per litre extra that the retailers are pocketing in margin so basically that whole £2.4bn cost to the Exchequer has gone straight to the bottom line of the petrol retailers," Ms Baldwin told the Today programme.
The competition watchdog has been investigating the UK fuel market following concerns that falling wholesale prices are not being passed on to consumers.
It said supermarkets were usually the cheapest place for fuel but competition was "not working as well as it should be".
It now plans to force supermarkets and other fuel retailers to publish live prices under a new scheme aimed at stopping them overcharging.
The BBC has contacted the Petrol Retailers Association for comment.