The recently appointed Independent Reviewer of Terrorism Legislation said hundreds of British jihadists who are returning from war zones pose a level of threat to the UK not seen since the Irish Republican Army (IRA) bombings in the 1970s.
Hill said the Islamic State was planning "indiscriminate attacks on innocent civilians on a scale similar to those perpetrated by the IRA 40 years ago".
"So I think that there is undoubtedly significant ongoing risk which is at least as great as the threat to London in the 1970s when the IRA were active on the mainland," he said.
More than 50 people were killed by IRA attacks in Britain during the 1970s.
"It's an enormous concern that large numbers - we know this means at least hundreds of British citizens who have left this country in order to fight - are now returning or may be about to return," he said.
Hill is tasked with scrutinising the UK's terror laws and report annually to the Parliament on the state of British terror legislation.
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