The warning was sounded by an industry body which charted a rise of more than 4p-per-litre in wholesale costs over the past five days alone.
The RAC said petrol was on course to overtake diesel costs for the first time in a year with average prices likely rising above 121p per litre - its highest level since 2014 - with no end to the surge currently in sight.
Diesel, it predicted, would remain stable at around 118.5p.
A spokesman for the motoring organisation, Pete Williams, said: "The price of unleaded petrol will leapfrog diesel early next week, rising by up to 4p per litre as the impact of Storm Harvey and the shut-down of large refineries in the Gulf Coast drives up US demand for petrol imports.
He added: "One quarter of the refining capacity in the States remains offline leaving a shortfall of over four billion barrels a day, and crude oil production in the Gulf of Mexico is down 13.5%.
"In response, oil traders are acting to redirect fuel to the US ahead of the busy Labor Day weekend surge in travel."
The PRA said it had been told by Platts, the independent provider of information and benchmark prices for the commodities and energy markets, that "there have been a large number of oil tankers set to move to the US from Europe carrying gasoline to make up shortfalls in the Gulf region".
PRA chairman Brian Madderson added: "This will be bad for UK inflation, for cash strapped UK motorists and for the economy as money is diverted from other purchases".