The average price of petrol fell by 6.69p a litre to 162.89p in September – the sixth biggest monthly drop since 2000 – saving drivers £3.68 a tank.

But data from RAC Fuel Watch also shows drivers should have seen a further 10p reduction had major retailers not upped their margins.

A 55-litre petrol fill-up has now dropped below the £90 mark for the first time since the start of May. This means that since unleaded peaked on 3 July at 191.53p, 28.64p has come off a litre, a saving of £15.75 on a full tank.

A litre of diesel also reduced in September but only by 3.5p a litre, finishing the month at 180.16p. This is 19p cheaper than its all-time high price of 199.09p, recorded on 25 June, leading to the cost of a tank falling £10.41 to £99.09.

The average price of unleaded at the big four supermarkets fell 6.6p a litre to 161.30p. But, unusually, this is only 1.5p lower than the UK average – normally it’s around 3.5p cheaper.

Supermarket diesel, however, only came down 1.4p to 178.56p which is 2p less than the UK average drop of 3.58p – diesel is also normally 3.5p cheaper at one of the big supermarkets.

The record high pump prices arose due to oil being trading around $130 a barrel in mid-June. At the end of September however, oil was back down to $87.96 – falling 5% in September alone.

‘Not playing fair with drivers’

Simon Williams, RAC fuel spokesman, said: “Despite September seeing the sixth biggest ever drop in the price of petrol, drivers really should have seen a far bigger drop as the wholesale price of delivered petrol was around 120p for the whole month. This means forecourts across the country should have been displaying prices around 152p given the long-term margin on unleaded is 7p a litre.

“In stark contrast to this, RAC Fuel Watch data has shown margins to be around 17p a litre – a huge 10p more than normal. And the average price of petrol at the big four supermarkets is only 1.5p lower than the UK average – less than half what it usually is which points heavily to them not playing fair with drivers.

“This is backed up by the fact that Morrisons, which was charging an average of 162p a litre for petrol at the end of September, is currently running a 5p a litre off promotion for customers spending £40 in store. This type of promotion tends only to be seen when supermarkets are benefitting from lower wholesale prices.”