But the Prime Minister told senior MPs that the evidence presented in support of the idea by his former catch-up tsar Kevan Collins was "not as good as it could have been".
Mr Johnson was pressed during an appearance in front of the Commons Liaison Committee about how funding to help children catch-up on months of lost education was being directed.
He said the loss of education over the last 18 months "has been for many, many children, really debilitating and there's no there's no question that they've lost unconscionable amounts of teaching and learning time".
Robert Halfon, the Conservative chair of the Education Select Committee, said the Government's £3bn catch-up funding so far had only reached 44 per cent of children on free school meals. And he said close to 100,000 'ghost children' had missed 50 per cent of school or more during the pandemic.
He asked: "Do you not think there should be a long term plan for education, a significant part of it would require a longer school day not just for academic catch up but for extracurricular activities such as sport and wellbeing?
"And what more needs to be done to convince you as Prime Minister and the Treasury that a fully-funded long-term plan with a longer school day is worth supporting and giving proper resources to?"
The plan drawn up by Sir Kevan Collins, the Government’s former education recovery commissioner, sought to establish a minimum 35-hour week and add 100 hours of additional schooling compared with existing timetables in many state schools and colleges.
Sir Kevan said last month he felt he had been left with no choice but to resign after the amount Ministers were prepared to invest fell so far short of what he believed was necessary.
The PM said today the Government was "looking at the evidence" about making the school day longer.
He said: "And if I'm absolutely frank with you and the committee, to begin with some of the evidence that was assembled was not as good as it could have been. The evidence on timetable, the evidence on lengthening the school day wasn't as powerful as it was on tuition for instance.
"But that doesn't mean it's not the right thing to do. I do think it's the right thing to do. The question is, how you do it, what sorts of activities, is it enrichment, is it academic, what's the mixture, we're doing a proper review of all of that to get the evidence that we want.
"But in the meantime, over the summer, there's all the summer schools, the holiday activities fund, a big effort to try to help kids to catch up before September as well."
He said that on top of £14bn already going into education, the Government had invested £3bn in its catch-up plan "plus anything else the Chancellor may do later this year".
He said: "I think the thing that works and the thing that I found that parents and teachers and pupils respond to is more direct one-to-one, one-to-two tuition and actually focusing on the needs of the individual child and drawing out where they're going wrong and really trying to focus on that."