On Boxing Day, the average price of petrol in London was 155p a litre while in Northern Ireland it was just 144.64p, according to the AA.
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That difference means a motorist filling up a 55-litre tank would pay £5 more in the capital than someone doing the same across the Irish Sea.
The motoring organisation also found that drivers in Wales and Scotland paid less for their fuel – 148.72p and 148.58p respectively.
The price difference for drivers of diesel cars was even wider.
In London a litre cost 178.17p while in Northern Ireland it was just 165.54p, the AA said.
That equates to a driver in Belfast paying nearly £7 less for a 55-litre tank of fuel than someone in London.
The AA found the cheapest petrol in Wales, as low as 136.9p a litre, in and around Milford Haven, Abergavenny, Abertillery and Ebbw Vale.
The gap in prices is far wider than the previous record in April 2020.
Then petrol was on average 7.1p cheaper in Northern Ireland, at 105.5p, compared with 112.6p in London.
In 2022, motorists have seen the price at the petrol forecourts jump by 32.5p a litre.
That increase added an average of £17.88 to the cost of filling up.
The price of diesel leapt by 42.4p this year, adding more than £23 to the price of an average tank of fuel.
With train strikes hitting over the run-up to Christmas, many people relied on their cars – and some petrol stations in London and the South East cashed in on this.
Luke Bosdet, of the AA, said: “Drivers pay far less for their fuel where competition is stimulated by an official price transparency tool… or a retailer sees a big opportunity to grab trade because prices are artificially high.
“The first has a widespread effect and can transform pricing across whole areas [and] the second creates a lottery where millions of drivers can miss out.
“Christmas and Boxing Day brought this to a head.
“Hundreds of thousands of travellers were forced from the railways into their cars to go home to see families, go out to shop or celebrate, or go to sporting events.
“This was probably a fuel sales bonanza for the forecourts.
“But while retailers in some areas of the UK slashed their pump prices because falls in wholesale costs allowed it, others cashed in on the Christmas travel misery.”
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