text: 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 isn'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.
target: 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 .
  Launches £100million 'prize' for firms who build first ultra low emission car .