The boss of energy company Cuadrilla said if the UK cannot develop its own oil and gas then it will be forced to import, putting the country at "risk in terms of security of supply."
Francis Egan told ITV News: "If we can't develop our own oil and gas then we have to import it. That costs billions of pounds, generates zero jobs and it puts us at risk in terms of security of supply."
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The Labour leader did not say whether he would maintain Britain's nuclear deterrent programme.
Ukip leader Paul Nuttall also said he would want to get rid of Sharia courts.
Over 40,000 people picked up their packs to take part in the annual marathon in the capital.