Britain’s security services face new questions over news that perpetrators in the Manchester and London Bridge attacks were “known wolves” — extremists who’d attracted public attention, even proclaiming themselves would-be jihadis.

Yet authorities considered none to be anything more than a “suspicious person” — and certainly not an imminent threat.

One London attacker, Khuram Shazad Butt, actually appeared last year in a TV documentary on “The Jihadis Next Door” unfurling an ISIS flag in a public park.

He also had close ties to one of the 7/7 transit terrorists as well as to a radical preacher known as a recruiter of British Muslims to extremism.

And local Muslims had actually reported his frightening views to officials.

Youssef Zaghba, meanwhile, was detained last year in Italy while trying to travel to Syria to fight for ISIS. He told a security guard he was “going to be a terrorist.”

He wasn’t charged due to “insufficient evidence.” Italy insists it flagged him to UK authorities; London says it never got the word.

The late Prime Minister Margaret Thatcher often noted that security forces need 100 percent success, while terrorists “only need to be lucky once.” But even Foreign Secretary Boris Johnson now asks, “How on earth could we have let this guy or possibly more through the net — what happened?”

Voters will consider that as they go to the polls Thursday.

Police say they’ve thwarted 18 jihadist attacks since 2013 — five in the past three months alone. But the sheer volume of the threat, and the number of potential jihadis, is clearly overwhelming.

Current resources may well be insufficient to remain fully vigilant. That will mean some uncomfortable choices. Is Britain — or America — prepared to make them?