David Prior, who was brought in to lead the Care Quality Commission in January after it faced criticism for failing to protect vulnerable patients, said that almost half of hospitals were now providing care which was either poor, or "not terribly" good.
He said the CQC had found 45 hospitals with problems dating back five years, and vowed that from now on regulators will take a "much clearer" approach in advising which hospitals should not be allowed to continue as they are.
We will be outside the system and the politics - we will have a huge role in the reconfiguration debate because we are independent.
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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.