“The lessons from last year’s tragic events must now result in real action,” the committee said.

Such is the scale of the challenge to fight terrorism, the authorities say, that 700 investigations are underway into some 3,000 suspects regarded as a serious threat, along with an additional 20,000 considered to be causing concern.

The police and MI5 say that they have thwarted 13 plots since March 2017, but the parliamentary panel focused on five attacks last year that claimed the lives of 36 victims.

The bloodiest of the attacks occurred on May 22, 2017, when Salman Abedi, a 22-year-old Briton of Libyan descent, blew himself up at a concert by Ariana Grande in the northwestern city of Manchester, killing 22 people, including several children.

Mr. Abedi had been known to the authorities because he had visited a jailed extremist in prison and had traveled to Libya from 2011 onward, the report said. But in 2015, the authorities judged him as posing a low risk.