Millions of petrol and diesel engine cars would be banned from the road within 25 years under Liberal Democrat plans . Only ultra low emission vehicles and electric cars would be allowed in the UK in 2040 under the proposal . Normal petrol and diesel engines which make up the vast majority of all motor vehicles would be barred . Scroll down for video . Liberal Democrat leader Nick Clegg takes a short break from campaigning and takes part in Go Ape Haldon Forest Park near Exeter today . Mr Clegg thanked 'Go Ape ' for having him this afternoon . He said it was 'healthy and active fun after being on the battle bus for so long ' As part of its 'green ' efforts , the party today announced a £100million 'prize ' for car companies who build the first ultra low emission vehicle . Lib Dem leader Nick Clegg said he wanted to tackle 'dangerous levels of air pollution ' which cut average life expectancy by up to eight months . He used a visit to Dymag , a specialist manufacturer of high performance racing wheels in Chippenham , Wiltshire , to highlight the plan to encourage greener motoring . To win the money , firms would have to create a family car which breaks into the top five in UK sales and have ultra low emissions ( Ulev ) . A Ulev is defined as a vehicle which has CO2 emissions of below 75g/km , far cleaner than the current 128.3g/km average for new cars . On current trends , becoming one of the five best selling cars in the UK would require sales of around 60,000 . Lib Dem leader Nick Clegg used a visit to Dymag , a specialist manufacturer of high performance racing wheels in Chippenham , Wiltshire , to highlight the plan to encourage greener motoring . Mr Clegg was joined by local Lib Dem candidate Duncan Hames , who was a ministerial aide to the Deputy PM . Mr Clegg said he wanted to tackle 'dangerous levels of air pollution ' which cut average life expectancy by up to eight months . Mr Clegg said : 'For the first time since the 1970s , Britain now sells more cars than it buys thanks to Vince Cable and the Liberal Democrats being in the driving seat at the Department for Business . 'However , we need to go further and faster in making Britain a world leader in low emission vehicles by providing the right incentives and infrastructure to attract green investment and green jobs . 'This is n't just about re-balancing the economy , it 's also about tackling dangerous levels of air pollution that reduce average life expectancy by up to eight months . 'Our Liberal Democrat £100 million prize fund will spark a low emission car revolution in Britain and help to build a stronger and greener economy for years to come . ' A party source said the prize would 'turbocharge the technology ' already being developed by manufacturers , incentivising them to bring models to market that would be affordable enough to become a best-seller . The party indicated that it expected a successful model would cost around £10,000 to £15,000 and would result in lower running costs for consumers .
Lib Dems want only ultra low emission vehicles and electric cars from 2040 . Clegg says he wanted to tackle 'dangerous levels of air pollution ' in cities . He also wants a ban on diesel vans, which are the most polluting form of car.