UK drivers were hit by one of the sharpest jumps in fuel prices in more than two decades last month, according to the RAC motoring organization.

The average cost of filling a tank climbed by roughly £4 ($5.05) in August, with unleaded petrol rising by almost 7 pence a liter and diesel by 8 pence, the RAC said in a statement. The increase could have been even higher had major retailers not allowed margins to return to more normal levels.

Pump prices were driven up by the cost of oil as producer group OPEC and its allies curbed output, the RAC said. That caused the wholesale cost of fuel — the price retailers pay – to go up, which they then passed on to consumers. The added expense comes as drivers in London face an expanded ultra low emissions zone, with a £12.50 charge on vehicles that don’t meet pollution standards.

“August was a big shock to drivers as they had grown used to seeing far lower prices than last summer’s record highs,” RAC fuel spokesman Simon Williams said. “Seeing £4 or more go on to the cost of a tank in the space of just a few weeks from a pump price rise of 6-7p a liter is galling, particularly for those who drive lots of miles or run an older, less fuel-efficient car.”

Read: London’s Emissions Zone Has Changed Air Pollution Policy Forever

Looking ahead, worries about the health of China’s economy and the end of the US summer driving season could potentially weigh on costs, the RAC said.