Boris Johnson told all petrol retailers on Friday to “pass on” to motorists the 5p a litre cut in fuel duty.
Downing Street stressed the reduction aimed to ease the cost-of-living crisis hitting many households across the country.
The Prime Minister’s official spokesman said: “We would ask everybody to pass on the cut to fuel duty.
“The Chancellor on Wednesday set out why we are introducing this measure to ease the burden on those people who need to fill up their cars or vans.
“So we would encourage absolutely everybody to pass on that fuel cut in duty to their customers.”
Transport Secretary Grant Shapps, earlier on Friday, called for consumer, as well as Government, pressure to be piled on petrol giants not to drag their feet in lowering prices to rake in more revenue.
Business Secretary Kwasi Kwarteng told the fuel firms to pass on the fuel duty cut within hours of it being announced on Wednesday in the Spring Statement.
Speaking on LBC Radio on Friday morning, Mr Shapps said: “Absolutely, pass this on.”
He also appealed: “Everyone has become very aware seeing £1.66, £1,70 for a litre of petrol…help us with this…shop around so that they learn that if they don’t pass this on, then they are going to lose business.
“So we can have consumer pressure, plus we will be putting maximum pressure on, not just as ministers but through the various different watchdogs to highlight who is and who is not passing that on.
“But if you are one of the majors, please make sure that you are passing this on.
“And I know that some of the supermarkets, the Asdas, Morrisons, Tesco and Sainsbury’s, they offer cheaper petrol….so do shop around.”
Mr Shapps later told ITV’S Good Morning Britain: “The price of petrol at £1.60, £1.70, is really hitting people hard.
“I will be speaking to the petrol retailers and asking them to make sure that they pass it (the 5p cut in fuel duty) on.
“Viewers know, you see big differentials, particularly between the supermarkets and petrol stations, please help us put pressure on them by shopping around as well, and not paying those higher prices, particularly where they fail to pass on this 5p cut.
“We will be watching very, very carefully and viewers can help.”
Petrol retailers have been accused of being quick to put up prices at the pump when wholesale costs rise and slower to cut them when buying fuel stocks fall in order to boost their profits.