Pain for motorists as filling car with petrol touches £100 He said the current high prices, inflated by concerns surrounding Iran and Syria, will only continue to rise. The duty increase in August will add a further £3.80 to the cost of filling a tank, Mr Watters said. He added that the recent rise meant the average motorist is paying £8.44 a month more to fill the car than at the start of the year, equivalent to £100 more a year. At a stroke, the increased cost wipes out almost half of the £220 average rise in personal tax allowances that the Chancellor announced in the Budget. "This will ram home the potential that fuel prices have in disrupting financial planning," Mr Watters said. Mr Osborne's decision to press ahead with a fuel duty rise in August has been met with disappointment. Brian Madderson, the chairman of RMI Petrol, the forecourt association, accused the Government of being "incredibly irresponsible." He said: "The increase will add 4p a litre from August when we already have a very volatile crude oil market. The Chancellor is gambling that the Iranian situation will sort itself out and push prices down. Petrol will probably hit 150p by this summer when the duty rise goes through. Neither retailers nor the voters want it. The general public does not have the money to pay for this. The RAC said the new high was "another painful barrier." Vince Cable, the Business Secretary, said yesterday that the Government had considered releasing state-held stockpiles of oil. However, he said: "We can't just sign vast cheques to solve all the problems of motorists." Oil was last night trading at about £78 ($125) a barrel.