Max Hill QC, the new Independent Reviewer of Terrorism Legislation, warned that Islamists' plots to attack UK cities presented an "enormous" risk.
But Mr Hill, who successfully prosecuted the failed 21/7 bombers, praised the intelligence services' "truly remarkable" success rate for foiling plots since the 2005 London bombings.
Mr Hill told the Sunday Telegraph: "I think the intensity and the potential frequency of the serious plot planning - with a view to indiscriminate attacks on innocent civilians of whatever race or colour in metropolitan areas - represents an enormous ongoing risk that none of us can ignore.
"So I think that there is undoubtedly significant ongoing risk which is at least as great as the threat to London in the 70s when the IRA were active on the mainland."
More than 50 people were killed by IRA attacks in Britain during the 1970s alone.
Isis is yet to land an attack in the UK but its militants have claimed lives in mainland Europe, including in the November 2015 Paris attacks that killed 130.
But Mr Hill, who will scrutinise UK's terror laws and produces reports, has questioned whether the fall of IS strongholds in the Middle East will cause British Islamists to return to the country to unleash terror.
In his interview, Mr Hill told the paper his proudest moment was putting the killers of Damilola Taylor behind bars.
He said: "Undoubtedly the killing of a 10-year-old boy on the streets of London when his Nigerian parents had sent him to be schooled in London thinking he would be safer here than at home, that is not only tragic but leaves a lasting impression.'
Announcing his appointment on Monday, Home Secretary Amber Rudd said that Mr Hill's "wealth of experience and legal expertise" would ensure the UK's terror laws were fair, necessary and proportionate.
