The Prime Minister insisted things looked good at “the moment" to proceed to step four, which would lift all legal restrictions and reopen nightclubs.
However, Mr Johnson also admitted the data was still too "ambiguous" to be certain.
Speaking on Wednesday, he said: “I can see nothing in the data at the moment that means we can’t go ahead with step four, all the reopening on June 21, but we’ve got to be so cautious.
“Because there’s no question the ONS [Office for National Statistics] data of infection rates is showing an increase.
"We always knew that was going to happen.
"What we need to work is out is to what extent the vaccination programme has protected enough of us, particularly the elderly and vulnerable, against a new surge.
“And there I’m afraid the data is just still ambiguous.
“And so every day we’re having long sessions, we’re interrogating all the data, we’re looking at all the various models, and the best the scientists can say at the moment in their guidance to us is we just need to give it a little bit longer.
“I’m sorry that’s frustrating for people. I know people want a clear answer about the way ahead for June 21, but at the moment we’ve just got to wait a little bit longer.”
Mr Johnson spoke as cases surge and the Delta Covid variant, formerly the Indian variant, has become dominant in the UK.
Despite this, there were zero deaths recorded in the UK on Monday for the first time since the pandemic began.
It comes just a day after Nicola Sturgeon confirmed several areas of Scotland would be kept in level two for an additional period of time in a bid to stave off a potential third wave.
The First Minister also confirmed Glasgow would move to level two as planned at midnight on Friday after more than eight months in some of the strictest restrictions in the UK.
Ministers had planned to move much of Scotland down to level one by Tuesday, with a further move to level zero by the end of the month, but this will now not go ahead.