In a live radio debate Home Secretary Ms Rudd suggested Mr Johnson had tried to “rush in” and attribute blame after it emerged the terrorist was not being watched despite having appeared in a TV programme on extremism.
Earlier in the day Foreign Secretary Mr Johnson claimed members of the public would read Tuesday's news headlines about Khuram Butt and think, "how on earth could we let this guy slip through the net?"
Ms Rudd, who is responsible for overseeing the security services, later responded saying: "It's not really for me, or for the Foreign Secretary, to say how that happened or what happened.
“Mark Rowley has already said that we will be looking at these elements of it. Like any incident you look and see if there's anything to learn. I can say that without putting any sort of suggestion of blame.
“It’s very easy to rush in and say what went wrong.”
The Conservatives were eager to move the election campaign back on to Brexit on Tuesday, but revelations about the London terror attacks continued to dominate the news – including that Butt was known to MI5 before he went on his rampage.
Butt had even appeared in a Channel 4 documentary, The Jihadis Next Door, about the presence of violent extremists in British cities. He was seen in the programme among a group of men unfurling a black Islamist flag and then in a confrontation with the police.
Mr Johnson said earlier on Tuesday morning: “People are going to look at the front pages today, and they are going to say how on Earth could we let this guy slip through the net?
“How could it possibly happen, how can he be on a Channel 4 programme and then committing atrocities like this?
“That is a question that will need to be answered by MI5 and by the police as the investigation goes on – I can’t answer that question now.”
There is still bad blood between the two Tory big-hitters after they clashed during the EU referendum debates, with Ms Rudd saying of Brexiteer Mr Johnson: “Boris, he's the life and soul of the party. But he’s not the man you want to drive you home at the end of the evening.”
Amid a row about figures, she then went on to add: “I fear that the only number Boris is interested in is No 10.”
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