He warned that housing and traffic will cover green fields unless something was done and described his message as a "call to action for everyone who cares about the future of rural England".
Mr Dimbleby, famed as a television presenter, was making a parting speech as president at the CPRE's general council. He said: "Three hundred people a day are leaving our major cities for the countryside, showing that not only have we lost confidence in our cities, but we are prepared to tolerate sprawl across our most precious environmental asset - our countryside.
"This is one of the central issues confronting the new Government, to which CPRE is committed to helping finding constructive solutions.
"But the big pressures facing the countryside are more powerful than ever, with 4.4 million new households projected to form by 2016, traffic on rural roads predicted to double or even treble by 2025 and a meagre 2 per cent of the huge agricultural budget devoted to environmentally friendly farming."
Praising the success of CPRE Mr Dimbleby said the organisation's efforts had led to:
out-of-town shopping centres losing favour.
cuts in the road programme.
the dropping of plans to relax curbs on outdoor advertising.
official policy to promote the sympathetic design of new development in the countryside.
renewed efforts to increasing the share of new housing in towns and cities.