PETROL prices have hit their highest level ever, leaving hard-pressed motorists paying more than £6 a gallon on average.

The latest in a series of rises, caused by concern about Iran, has put Chancellor George Osborne under pressure to scrap a 3.02p fuel duty increase due in August.

As motoring organisations reacted with anger to the revelation petrol was past the £1.40-per litre barrier – the equivalent of £6.36 a gallon – it emerged communities in the north of Scotland and the Northern Isles are already paying £1.48 per litre.

It comes amid fears that fuel costing £1.50 per litre could be found on garage forecourts, pricing people out of using essential transport.

David Bizley, the RAC's technical director, said: "A figure of £1.40 a litre is a massive price for people to have to pay and there is no end in sight to rising prices.

"The way things are going, the planned duty rise will see average petrol prices hit the £1.50 a litre mark – forcing more and more people who need their cars off the road."

Mr Osborne has faced calls to end the "highway robbery" of motorists after the AA revealed the petrol rises. Diesel is also at a new record of 146.72p per litre.

Quentin Willson, national spokesman for campaign group FairFuelUK, said: "Mr Osborne could have acted to cushion the UK economy, but he ignored our warnings and the desperate pleas of the tens of millions of people who can't cope with the highest cost of petrol and diesel ever seen in this country."

The AA revealed that, for a car consuming 106.17 litres of petrol a month, the 7.95p-per-litre rise seen so far in 2012 has added £8.44 to the monthly bill.

The row came as Business Secretary Vince Cable confirmed the UK Government was considering releasing some of its emergency stockpiles of oil in a bid to push down prices.

Labour's Shadow Treasury Minister, Cathy Jamieson, accused the Government of hitting those already struggling.

She said: "It shows just how out of touch George Osborne is that in this week's Budget he said he could afford to give a huge tax cut to people earning over £150,000, but couldn't afford to cut fuel duty for middle and low income families feeling the squeeze."

Labour leader Ed Miliband denounced the Budget as a "disastrous" Budget for millionaires – a Budget which will hurt pensioners, families and hard-pressed working people".

He called on voters to make next week's Westminster by-election in Bradford, in which former Glasgow MP George Galloway is standing, a verdict on the Budget, amid the continuing outcry over the so-called "granny tax" and tax cuts for millionaires.

The SNP's Angus MacNeil said: "David Cameron needs to stop dithering and start delivering action to bring soaring fuel prices down.

"With the bulk of the pump price made up of tax, the Treasury must stop this highway robbery because soaring fuel prices are hindering economic recovery."

Mr Cable confirmed ministers were involved in global negotiations over potentially releasing part of the UK's emergency oil stockpiles to stem rises.

Prices vary widely across Scotland and this has prompted periodical pockets of protest, mainly in rural and island areas where costs of fuel transportation have been cited for higher charges. They range from 135.9p to 142.9p for petrol and 142.9p to 146.9p for diesel in Renfrewshire, and 133.9p to 141.9p in Dundee for petrol and 141.9p to 146.9p for diesel.

In Shetland, prices are about 148.9p for petrol and 154.9p for diesel.

A protest in Shetland this week saw one garage offer petrol for 129.9p and diesel for 135.9p, around 20p per litre less than usual. Scott and Phoebe Preston of Tagon Stores, in Voe, sold 10,000 litres at reduced prices.